[1.1.X] Fixed #13184 -- Document the requirement that custom model fields...

[1.1.X] Fixed #13184 -- Document the requirement that custom model fields using SubfieldBase should probably implement formfield. Thanks to Mark L. for the report, and to Joseph and Honza for the guidance on the solution.

Backport of r12979 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12980 bcc190cf-cafb-0310-a4f2-bffc1f526a37