lib: Invalidate message metadata in _notmuch_message_gen_terms

Previously, we invalidated stored message metadata in
_notmuch_message_add_term and _notmuch_message_remove_term, but not in
_notmuch_message_gen_terms.  This doesn't currently result in any bugs
because of our limited uses of _notmuch_message_gen_terms, but it may
could cause trouble in the future.
lib: Fix slight misinformation in the database schema doc

The database schema documentation made it sound like each mail
document had exactly one on-disk message file, which hasn't been true
for a long time.