The -config manpage is a bit long and the -watch stuff is
isolated from the rest of it while we start documenting NNTP and
IMAP support.
I'm not entirely happy with the way IMAP and NNTP are
configured, it's still good enough for small setups.
This also fixes a long-standing misplaced comment about
`publicinboxwatch.spamcheck' affecting all configured inboxes,
that comment was actually for `publicinboxwatch.watchspam'.
We'll omit documenting NNTP for `watchspam', for now, given the
lack of \Seen flags in NNTP and I'm not sure if it's even
useful. There may not be any newsgroups for sharing confirmed
spam, either...