[1.1.X] Created a 'DB optimization' topic, with cross-refs to relevant sections.

Also fixed #10291, which was related, and cleaned up some inconsistent doc labels.

Backport of r12229 from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12230 bcc190cf-cafb-0310-a4f2-bffc1f526a37