Fixed #11882 -- Added documentation for formfield_for_manytomany. Thanks to...

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

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13552 bcc190cf-cafb-0310-a4f2-bffc1f526a37