📚 Update IMAP#search docs again

Now that rdoc 6.8 renders properly on small screens, the IMAP#search
docs needed to be updated.  Nested unordered lists indent too deeply for
small screens.

Also, the search rdoc reads better when the details of argument
translation are moved from the main text into its own subsection.

Also, the search key reference has been reorganized around key type.

Also, the heading levels were all adjusted to `h4` and `h5`.  This
assumes that the method heading is an `h3` heading.  Unfortunately, the
method headings use a `<div>`, but they _should_ be `h3` headings. 😉

lib/net/imap.rb

@@ -1940,28 +1940,8 @@ def uid_expunge(uid_set)
     #
     # +criteria+ is one or more search keys and their arguments, which may be
     # provided as an array or a string.
-    # See {"Search criteria"}[rdoc-ref:#search@Search+criteria], below.
-    #
-    # * When +criteria+ is an array, each member is a +SEARCH+ command argument:
-    #   * Any SequenceSet sends SequenceSet#valid_string.
-    #     These types are converted to SequenceSet for validation and encoding:
-    #     * +Set+
-    #     * +Range+
-    #     * <tt>-1</tt> and +:*+ -- both translate to <tt>*</tt>
-    #     * responds to +#to_sequence_set+
-    #     * +Array+, when each element is one of the above types, a positive
-    #       +Integer+, a sequence-set formatted +String+, or a deeply nested
-    #       +Array+ of these same types.
-    #   * Any +String+ is sent verbatim when it is a valid \IMAP atom,
-    #     and encoded as an \IMAP quoted or literal string otherwise.
-    #   * Any other nested +Array+ is encoded as a parenthesized list, to group
-    #     multiple search keys (e.g., for use with +OR+ and +NOT+).
-    #   * Any other +Integer+ (besides <tt>-1</tt>) will be sent as +#to_s+.
-    #   * +Date+ objects will be encoded as an \IMAP date (see ::encode_date).
-    #
-    # * When +criteria+ is a string, it will be sent directly to the server
-    #   <em>without any validation or encoding</em>.  *WARNING:* This is
-    #   vulnerable to injection attacks when external inputs are used.
+    # See {"Argument translation"}[rdoc-ref:#search@Argument+translation]
+    # and {"Search criteria"}[rdoc-ref:#search@Search+criteria], below.
     #
     # +charset+ is the name of the {registered character
     # set}[https://www.iana.org/assignments/character-sets/character-sets.xhtml]
@@ -1972,7 +1952,7 @@ def uid_expunge(uid_set)
     #
     # Related: #uid_search
     #
-    # ===== For example:
+    # ==== For example:
     #
     #   p imap.search(["SUBJECT", "hello", "NOT", "SEEN"])
     #   #=> [1, 6, 7, 8]
@@ -1988,7 +1968,45 @@ def uid_expunge(uid_set)
     #    # criteria string contains charset arg
     #    imap.search("CHARSET UTF-8 OR UNSEEN (FLAGGED SUBJECT foo)")
     #
-    # ===== Search keys
+    # ==== Argument translation
+    #
+    # [When +criteria+ is an Array]
+    #   Each member is a +SEARCH+ command argument:
+    #   [SequenceSet]
+    #     Encoded as an \IMAP +sequence-set+ with SequenceSet#valid_string.
+    #   [Set, Range, <tt>-1</tt>, +:*+, responds to +#to_sequence_set+]
+    #     Converted to SequenceSet for validation and encoding.
+    #   [nested sequence-set +Array+]
+    #     When every element in a nested array is one of the above types, a
+    #     positive +Integer+, a sequence-set formatted +String+, or a deeply
+    #     nested +Array+ of these same types, the array will be converted to
+    #     SequenceSet for validation and encoding.
+    #   [Any other nested +Array+]
+    #     Otherwise, a nested array is encoded as a parenthesized list, to
+    #     combine multiple search keys (e.g., for use with +OR+ and +NOT+).
+    #   [+String+]
+    #     Sent verbatim when it is a valid \IMAP +atom+, and encoded as an \IMAP
+    #     +quoted+ or +literal+ string otherwise.  Every standard search key
+    #     name is a valid \IMAP +atom+ and every standard search key string
+    #     argument is an +astring+ which may be encoded as +atom+, +quoted+, or
+    #     +literal+.
+    #
+    #     *Note:* <tt>*</tt> is not a valid \IMAP +atom+ character.  Any string
+    #     containing <tt>*</tt> will be encoded as a +quoted+ string, _not_ a
+    #     +sequence-set+.
+    #   [+Integer+ (except for <tt>-1</tt>)]
+    #     Encoded using +#to_s+.
+    #   [+Date+]
+    #     Encoded as an \IMAP date (see ::encode_date).
+    #
+    # [When +criteria+ is a String]
+    #   +criteria+ will be sent directly to the server <em>without any
+    #   validation or encoding</em>.
+    #
+    #   <em>*WARNING:* This is vulnerable to injection attacks when external
+    #   inputs are used.</em>
+    #
+    # ==== Search keys
     #
     # For full definitions of the standard search +criteria+,
     # see [{IMAP4rev1 §6.4.4}[https://www.rfc-editor.org/rfc/rfc3501.html#section-6.4.4]],
@@ -2003,145 +2021,161 @@ def uid_expunge(uid_set)
     # arguments.  The number and type of arguments is specific to each search
     # key.
     #
-    # +ALL+::
-    #   Matches every message in the mailbox.
+    # ===== Search keys that match all messages
     #
-    # (_search-key_ _search-key_...)::
-    #   Combines one or more _search-key_ arguments to match
-    #   messages which match all contained search keys.  Useful for +OR+, +NOT+,
-    #   and other search keys with _search-key_ arguments.
+    # [+ALL+]
+    #   The default initial key.  Matches every message in the mailbox.
     #
-    #   _Note:_ this search key has no label.
+    # [+SAVEDATESUPPORTED+]
+    #   Matches every message in the mailbox when the mailbox supports the save
+    #   date attribute.  Otherwise, it matches no messages.
     #
-    # +OR+ _search-key_ _search-key_::
-    #   Matches messages which match either _search-key_ argument.
+    #   <em>Requires +SAVEDATE+ capability</em>.
+    #   {[RFC8514]}[https://www.rfc-editor.org/rfc/rfc8514.html#section-4.3]
     #
-    # +NOT+ _search-key_::
-    #   Matches messages which do not match _search-key_.
+    # ===== Sequence set search keys
     #
-    # _sequence-set_::
+    # [_sequence-set_]
     #   Matches messages with message sequence numbers in _sequence-set_.
     #
     #   _Note:_ this search key has no label.
     #
     #   <em>+UIDONLY+ must *not* be enabled.</em>
     #   {[RFC9586]}[https://www.rfc-editor.org/rfc/rfc9586.html]
     #
-    # +UID+ _sequence-set_::
+    # [+UID+ _sequence-set_]
     #   Matches messages with a UID in _sequence-set_.
     #
-    # +ANSWERED+::
-    # +UNANSWERED+::
+    # ===== Compound search keys
+    #
+    # [(_search-key_ _search-key_...)]
+    #   Combines one or more _search-key_ arguments to match
+    #   messages which match all contained search keys.  Useful for +OR+, +NOT+,
+    #   and other search keys with _search-key_ arguments.
+    #
+    #   _Note:_ this search key has no label.
+    #
+    # [+OR+ _search-key_ _search-key_]
+    #   Matches messages which match either _search-key_ argument.
+    #
+    # [+NOT+ _search-key_]
+    #   Matches messages which do not match _search-key_.
+    #
+    # [+FUZZY+ _search-key_]
+    #   Uses fuzzy matching for the specified search key.
+    #
+    #   <em>Requires <tt>SEARCH=FUZZY</tt> capability.</em>
+    #   {[RFC6203]}[https://www.rfc-editor.org/rfc/rfc6203.html#section-6].
+    #
+    # ===== Flags search keys
+    #
+    # [+ANSWERED+, +UNANSWERED+]
     #   Matches messages with or without the <tt>\\Answered</tt> flag.
-    # +DELETED+::
-    # +UNDELETED+::
+    # [+DELETED+, +UNDELETED+]
     #   Matches messages with or without the <tt>\\Deleted</tt> flag.
-    # +DRAFT+::
-    # +UNDRAFT+::
+    # [+DRAFT+, +UNDRAFT+]
     #   Matches messages with or without the <tt>\\Draft</tt> flag.
-    # +FLAGGED+::
-    # +UNFLAGGED+::
+    # [+FLAGGED+, +UNFLAGGED+]
     #   Matches messages with or without the <tt>\\Flagged</tt> flag.
-    # +SEEN+::
-    # +UNSEEN+::
+    # [+SEEN+, +UNSEEN+]
     #   Matches messages with or without the <tt>\\Seen</tt> flag.
-    #
-    # +KEYWORD+ _keyword_::
-    # +UNKEYWORD+ _keyword_::
+    # [+KEYWORD+ _keyword_, +UNKEYWORD+ _keyword_]
     #   Matches messages with or without the specified _keyword_.
     #
-    # +BCC+ _substring_::
-    #   Matches when _substring_ is in the envelope's BCC field.
-    # +CC+ _substring_::
-    #   Matches when _substring_ is in the envelope's CC field.
-    # +FROM+ _substring_::
-    #   Matches when _substring_ is in the envelope's FROM field.
-    # +SUBJECT+ _substring_::
-    #   Matches when _substring_ is in the envelope's SUBJECT field.
-    # +TO+ _substring_::
-    #   Matches when _substring_ is in the envelope's TO field.
-    #
-    # +HEADER+ _field_ _substring_::
+    # [+RECENT+, +UNRECENT+]
+    #   Matches messages with or without the <tt>\\Recent</tt> flag.
+    #
+    #   *NOTE:* The <tt>\\Recent</tt> flag has been removed from +IMAP4rev2+.
+    # [+NEW+]
+    #   Equivalent to <tt>(RECENT UNSEEN)</tt>.
+    #
+    #   *NOTE:* The <tt>\\Recent</tt> flag has been removed from +IMAP4rev2+.
+    #
+    # ===== Header field substring search keys
+    #
+    # [+BCC+ _substring_]
+    #   Matches when _substring_ is in the envelope's +BCC+ field.
+    # [+CC+ _substring_]
+    #   Matches when _substring_ is in the envelope's +CC+ field.
+    # [+FROM+ _substring_]
+    #   Matches when _substring_ is in the envelope's +FROM+ field.
+    # [+SUBJECT+ _substring_]
+    #   Matches when _substring_ is in the envelope's +SUBJECT+ field.
+    # [+TO+ _substring_]
+    #   Matches when _substring_ is in the envelope's +TO+ field.
+    #
+    # [+HEADER+ _field_ _substring_]
     #   Matches when _substring_ is in the specified header _field_.
     #
-    # +BODY+ _string_::
+    # ===== Body text search keys
+    # [+BODY+ _string_]
     #   Matches when _string_ is in the body of the message.
     #   Does not match on header fields.
     #
     #   The server _may_ use flexible matching, rather than simple substring
     #   matches.  For example, this may use stemming or match only full words.
     #
-    # +TEXT+ _string_::
+    # [+TEXT+ _string_]
     #   Matches when _string_ is in the header or body of the message.
     #
     #   The server _may_ use flexible matching, rather than simple substring
     #   matches.  For example, this may use stemming or match only full words.
     #
-    # +BEFORE+ _date_::
-    # +ON+ _date_::
-    # +SINCE+ _date_::
-    #   Matches when the +INTERNALDATE+ is earlier than, on, or later than
-    #   _date_.
+    # ===== Date/Time search keys
     #
-    # +SENTBEFORE+ _date_::
-    # +SENTON+ _date_::
-    # +SENTSINCE+ _date_::
+    # [+SENTBEFORE+ _date_]
+    # [+SENTON+ _date_]
+    # [+SENTSINCE+ _date_]
     #   Matches when the +Date+ header is earlier than, on, or later than _date_.
     #
-    # +SMALLER+ _bytes_::
-    # +LARGER+ _bytes_::
-    #   Matches when +RFC822.SIZE+ is smaller/larger than _bytes_.
-    #
-    # ====== Removed from +IMAP4rev2+
-    #
-    # The <tt>\\Recent</tt> flag has been removed from +IMAP4rev2+.  So these
-    # search keys require the +IMAP4rev1+ capability.
+    # [+BEFORE+ _date_]
+    # [+ON+ _date_]
+    # [+SINCE+ _date_]
+    #   Matches when the +INTERNALDATE+ is earlier than, on, or later than
+    #   _date_.
     #
-    # +RECENT+::
-    # +UNRECENT+::
-    #   Matches messages with or without the <tt>\\Recent</tt> flag.
+    # [+OLDER+ _interval_]
+    # [+YOUNGER+ _interval_]
+    #   Matches when the +INTERNALDATE+ is more/less than _interval_ seconds ago.
     #
-    # +NEW+::
-    #   Equivalent to <tt>(RECENT UNSEEN)</tt>.
+    #   <em>Requires +WITHIN+ capability</em>.
+    #   {[RFC5032]}[https://www.rfc-editor.org/rfc/rfc5032.html]
     #
-    # ====== Extension search keys
+    # [+SAVEDBEFORE+ _date_]
+    # [+SAVEDON+ _date_]
+    # [+SAVEDSINCE+ _date_]
+    #   Matches when the save date is earlier than, on, or later than _date_.
     #
-    # The search keys described below are defined by standard \IMAP extensions.
+    #   <em>Requires +SAVEDATE+ capability.</em>
+    #   {[RFC8514]}[https://www.rfc-editor.org/rfc/rfc8514.html#section-4.3]
     #
-    # +OLDER+ _interval_::
-    # +YOUNGER+ _interval_::
-    #   Matches when +INTERNALDATE+ is more/less than _interval_ seconds ago.
+    # ===== Other message attribute search keys
     #
-    #   <em>Requires the +WITHIN+ capability</em>.
-    #   {[RFC5032]}[https://www.rfc-editor.org/rfc/rfc5032.html]
+    # [+SMALLER+ _bytes_]
+    # [+LARGER+ _bytes_]
+    #   Matches when +RFC822.SIZE+ is smaller or larger than _bytes_.
     #
-    # +ANNOTATION+ _entry_ _attr_ _value_::
+    # [+ANNOTATION+ _entry_ _attr_ _value_]
     #   Matches messages that have annotations with entries matching _entry_,
     #   attributes matching _attr_, and _value_ in the attribute's values.
     #
-    #   <em>Requires the +ANNOTATE-EXPERIMENT-1+ capability</em>.
+    #   <em>Requires +ANNOTATE-EXPERIMENT-1+ capability</em>.
     #   {[RFC5257]}[https://www.rfc-editor.org/rfc/rfc5257.html].
     #
-    # +FILTER+ _filter_::
+    # [+FILTER+ _filter_]
     #   References a _filter_ that is stored on the server and matches all
     #   messages which would be matched by that filter's search criteria.
     #
-    #   <em>Requires the +FILTERS+ capability</em>.
+    #   <em>Requires +FILTERS+ capability</em>.
     #   {[RFC5466]}[https://www.rfc-editor.org/rfc/rfc5466.html#section-3.1]
     #
-    # +FUZZY+ _search-key_::
-    #   Uses fuzzy matching for the specified search key.
-    #
-    #   <em>Requires the <tt>SEARCH=FUZZY</tt> capability.</em>
-    #   {[RFC6203]}[https://www.rfc-editor.org/rfc/rfc6203.html#section-6].
-    #
-    # +MODSEQ+ _modseq_::
+    # [+MODSEQ+ _modseq_]
     #   Matches when +MODSEQ+ is greater than or equal to _modseq_.
     #
-    #   <em>Requires the +CONDSTORE+ capability</em>.
+    #   <em>Requires +CONDSTORE+ capability</em>.
     #   {[RFC7162]}[https://www.rfc-editor.org/rfc/rfc7162.html#section-3.1.5].
     #
-    # +MODSEQ+ _entry_ _entry-type_ _modseq_::
+    # [+MODSEQ+ _entry_ _entry-type_ _modseq_]
     #   Matches when a specific metadata _entry_ has been updated since
     #   _modseq_.
     #
@@ -2150,33 +2184,18 @@ def uid_expunge(uid_set)
     #   <tt>\\</tt> prefix.  _entry-type_ can be one of <tt>"shared"</tt>,
     #   <tt>"priv"</tt> (private), or <tt>"all"</tt>.
     #
-    #   <em>Requires the +CONDSTORE+ capability</em>.
+    #   <em>Requires +CONDSTORE+ capability</em>.
     #   {[RFC7162]}[https://www.rfc-editor.org/rfc/rfc7162.html#section-3.1.5].
     #
-    # +EMAILID+ _objectid_::
-    # +THREADID+ _objectid_::
+    # [+EMAILID+ _objectid_]
+    # [+THREADID+ _objectid_]
     #   Matches when +EMAILID+/+THREADID+ is equal to _objectid_
     #   (substring matches are not supported).
     #
-    #   <em>Requires the +OBJECTID+ capability</em>.
+    #   <em>Requires +OBJECTID+ capability</em>.
     #   {[RFC8474]}[https://www.rfc-editor.org/rfc/rfc8474.html#section-6]
     #
-    # +SAVEDATESUPPORTED+::
-    #   Matches every message in the mailbox when the mailbox supports the save
-    #   date attribute.  Otherwise, it matches no messages.
-    #
-    #   <em>Requires the +SAVEDATE+ capability</em>.
-    #   {[RFC8514]}[https://www.rfc-editor.org/rfc/rfc8514.html#section-4.3]
-    #
-    # +SAVEDBEFORE+ _date_::
-    # +SAVEDON+ _date_::
-    # +SAVEDSINCE+ _date_::
-    #   Matches when the save date is earlier than, on, or later than _date_.
-    #
-    #   <em>Requires the +SAVEDATE+ capability.</em>
-    #   {[RFC8514]}[https://www.rfc-editor.org/rfc/rfc8514.html#section-4.3]
-    #
-    # ===== Capabilities
+    # ==== Capabilities
     #
     # If CONDSTORE[https://www.rfc-editor.org/rfc/rfc7162.html] is supported
     # and enabled for the selected mailbox, a non-empty SearchResult will