Fixed #13211 -- Added the `Group` API reference and a `Permission` API example...

Fixed #13211 -- Added the `Group` API reference and a `Permission` API example to the `contrib.auth` documentation. Thanks to b14ck for the report and to jpaulett and CrazyGir for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16849 bcc190cf-cafb-0310-a4f2-bffc1f526a37