Fixed #8967: Added documentation for the 'through' option on a many-to-many...

Fixed #8967: Added documentation for the 'through' option on a many-to-many field in the fields reference documentation.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@9083 bcc190cf-cafb-0310-a4f2-bffc1f526a37