[1.2.X] Fixed #11047 -- Clarified the explanation of arguments to...

[1.2.X] Fixed #11047 -- Clarified the explanation of arguments to GenericForeignKey. Thanks to psmith and timo for their work on the patch.

Backport of r13554 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13561 bcc190cf-cafb-0310-a4f2-bffc1f526a37