Refs #12767 -- Modified the multi-db docs to remove the implication that...

Refs #12767 -- Modified the multi-db docs to remove the implication that contrib.auth can be easily put on a separate database.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12751 bcc190cf-cafb-0310-a4f2-bffc1f526a37