`dt:' documentation is redundant with `d:' approxidate support;
so drop `dt:' since mairix uses `d:'. We'll also document
`rt:' since there are legit messages from senders with broken
clocks.
Reduce indentation level of help texts to be in 2-space
increments to using too much horizontal space.
We'll always place IMAP ahead of NNTP since it's alphabetical
and there's likely more IMAP clients out there.
Add "--ng NEWSGROUP" to -init instructions if configured.
There's also some minor wording changes throughout.