Clarify IMAP MIME body sections, which are confusing because every

Clarify IMAP MIME body sections, which are confusing because every
message body is treated as a multipart body by the IMAP's indexing
scheme.  This makes IMAIL never fetch TEXT body parts, except when
the user views a message raw with `C-c C-t C-m' (eventually, which
will also view arbitrary MIME bodies raw), and rather use numbered
parts, which will cause IMAIL to refill existing disk caches, even
though they already have mostly the same data in them.  IMAIL will
also now show MIME bodies in IMAP and file folders more uniformly,
especially complex nesting of message/rfc822 and multipart bodies.