mail-filter/spamassassin: update description of USE=bayes flag.

mail-filter/spamassassin: update description of USE=bayes flag.

A user asked us to enable Redis support for SpamAssassin in bug
617950, but it should already be there. SpamAssassin has its own
"TinyRedis" client, and doesn't pull in any extra dependencies to
enable it.

This is somewhat confusing when you look at the REQUIRED_USE for
SpamAssassin, which suggests that, to enable USE=bayes, you need to
pick a particular database. I only see three solutions, none of which
are great:

  1. Introduce a USE=redis flag that does nothing.

  2. Try to document the USE=bayes flag better, to make it clear
     that you get Redis support even with USE="-bayes".

  3. Allow USE=bayes without picking a database driver.

The first I've ruled out for aesthetic reasons, and the third I think
could do more harm than good (to everyone who wants bayes without
Redis). Therefore I've taken the second option, and updated the
documentation for the local USE=bayes flag. It now mentions that you
don't need USE=bayes to get the Redis bayes backend -- it's always
there.

Gentoo-Bug: 617950

Package-Manager: Portage-2.3.3, Repoman-2.3.1