Fixed #16742 -- Provided code examples in the models documentation for...

Fixed #16742 -- Provided code examples in the models documentation for accessing extra fields on many-to-many relationships. Many thanks to aj for the suggestion and to Nick Lang for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16817 bcc190cf-cafb-0310-a4f2-bffc1f526a37