Mention when last deck/notetype is saved

https://forums.ankiweb.net/t/default-deck-per-note-type/41230/3

src/preferences.md

@@ -130,6 +130,9 @@ note type (and opens the add window to the last-used note type when you choose
 Add). This may be more convenient if you always use a single note type for each
 deck.
 
+The last used deck/notetype is updated when you add a card. If you change the deck
+and close the add window without adding a card, it won't be saved.
+
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