lib: Document that absence of maildir flags can also remove/add tags

Previously the documentation of notmuch_message_maildir_flags_to_tags
suggested that the presence of a flag would cause tags to be added,
(or in the case of "unread", removed). But the case of absent maildir
flags was not explicitly described.

What we actually want, is that for supported flags, the absence of the
flag in all messages causes the corresponding tag to be removed,
(or in the case of "unread", added). So document that explicitly.

This is the case recently added to the test suite as a failing test,
(so we'll need to do bug fixing before the documentation is honest
here).

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