Clarify the documentation around SQLite and case-sensitive string matching.

This was still causing some confusion, so I rewrote the section in the
database notes to encompass both substring matching and non-ASCII
case-insensitive equality checks, as well as putting in a stronger
callout on the "contains" filter.

Refs #16569.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16694 bcc190cf-cafb-0310-a4f2-bffc1f526a37