Added documentation to explain the gains and losses when using utf8_bin

collation in MySQL. This should help people to make a reasonably informed
decision. Usually, leaving the MySQL collation alone will be the best solution,
but if you must change it, this gives a start to the information you need and
pointers to the appropriate place in the MySQL docs.

There's a small chance I also got all the necessary Sphinx markup correct, too
(it builds without errors, but I may have missed some chances for glory and
linkage).

Fixed #2335, #8506.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@8568 bcc190cf-cafb-0310-a4f2-bffc1f526a37