Update Clipboard toolkit documentation

Re: #77.

The Clipboard toolkit documentation section now includes
information on the cross-platform Clipboard class as well as which
Clipboard class is the default on which platform.

documentation/clipboard.txt

@@ -11,6 +11,11 @@ this toolkit.  Each instance of this class is a container with a structure
 similar to the Windows system clipboard, mapping content formats to
 content data.
 
+The :class:`dragonfly.util.clipboard.Clipboard` can be used on other
+platforms, such as macOS or Linux. Its API is compatible with the Windows
+clipboard class **except** for the :meth:`has_format`, :meth:`get_format`
+and :meth:`set_format` methods, as the class doesn't support multiple
+formats.
 
 Usage examples
 ----------------------------------------------------------------------------
@@ -61,14 +66,21 @@ achieved as follows: ::
    Clipboard(unicode=u'asdf', text, oemtext, locale)
 
 
-Clipboard class
+Windows Clipboard class
 ----------------------------------------------------------------------------
 
 .. autoclass:: dragonfly.windows.clipboard.Clipboard
    :members:
 
 
-Clipboard context manager function
+Cross-platform Clipboard class
+----------------------------------------------------------------------------
+
+.. autoclass:: dragonfly.util.clipboard.Clipboard
+   :members:
+
+
+Windows Clipboard context manager function
 ----------------------------------------------------------------------------
 
 .. autofunction:: dragonfly.windows.clipboard.win32_clipboard_ctx

dragonfly/util/clipboard.py

@@ -54,11 +54,20 @@ def get_text(self):
 
 class Clipboard(BaseClipboard):
     """
-    This class provides multi-platform clipboard support through pyperclip.
+    Cross-platform class for interacting with the system clipboard via the
+    `pyperclip <https://pyperclip.readthedocs.io/en/latest/>`__ Python
+    package.
+
+    This is Dragonfly's default clipboard class on platforms other than
+    Windows.
+
+    .. note::
+
+       This class does work on Windows, however the Windows
+       :class:`dragonfly.windows.clipboard.Clipboard` class should be used
+       instead because this class only currently supports the Unicode text
+       clipboard format.
 
-    The only currently supported Windows clipboard format is Unicode text.
-    The Clipboard class in dragonfly.windows.clipboard can be used instead
-    if required.
     """
 
     # ----------------------------------------------------------------------

dragonfly/windows/clipboard.py

@@ -87,6 +87,9 @@ def win32_clipboard_ctx():
 class Clipboard(BaseClipboard):
     """
     Class for interacting with the Windows system clipboard.
+
+    This is Dragonfly's default clipboard class on Windows.
+
     """
 
     #-----------------------------------------------------------------------