www: various help text updates

`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.

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