[1.2.X] Fixed #11882 -- Added documentation for formfield_for_manytomany....

[1.2.X] Fixed #11882 -- Added documentation for formfield_for_manytomany. Thanks to Rob Hudson, timo and Simon Meers for their work on the patch.

Backport of r13552 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13559 bcc190cf-cafb-0310-a4f2-bffc1f526a37