Fixed #12837 -- Clarified the naming strategy for m2m intermediate tables....

Fixed #12837 -- Clarified the naming strategy for m2m intermediate tables. Thanks to Ramiro Morales for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12516 bcc190cf-cafb-0310-a4f2-bffc1f526a37