Fixed #14820 -- Added more information to the generic relation docs regarding...

Fixed #14820 -- Added more information to the generic relation docs regarding different choices for storing PK references for a GenericForeignKey. Thanks to mrmachine for the all the work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15545 bcc190cf-cafb-0310-a4f2-bffc1f526a37