[1.2.X] Fixed #12313 - Add a note that QuerySet.delete() doesn't necessarily...

[1.2.X] Fixed #12313 - Add a note that QuerySet.delete() doesn't necessarily call obj.delete(). thanks FunkyELF for the suggestion.

Backport of r15071 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15072 bcc190cf-cafb-0310-a4f2-bffc1f526a37