Fixed #13452 - Document that regroup can be used with any template lookup. ...

Fixed #13452 - Document that regroup can be used with any template lookup.  Thanks shacker for the suggestion and draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@15108 bcc190cf-cafb-0310-a4f2-bffc1f526a37