Update pattern documentation.

Update pattern documentation.

Note that ~b, ~B, ~h, ~M, and ~X read the message in, and that this
can be much slower.

Change =b, =B, =h table rows to document just the IMAP behavior.  The
"=" simple string behavior is already covered below, and makes it less
obvious whether "***)" should be added to those entries in the table.

Break up the simple string and imap server-side matching explanation
into two paragraphs.

Add ~M to the list of patterns you can't use message scoring with.

Add equivalent, if briefer, explanations to the muttrc.man page.