notmuch new, restore, and tag commands support maildir flag
synchronization with notmuch tags. Reference the notmuch-config(1) man
page about it in the relevant man pages.
These functions are enough different in their behavior that it's not
really worth it to combine them. They overlap in the format of the
dump file, but we can have a separate page that describes the dump
format, and either reference it or include it. This also keeps things
nice and clean with one page per command.
These are just for the convenience of testers using $src/man as an
element of MANPATH. They are intentionally omitted from MAN1, so that
they do not interfere with the install.
