[1.2.X] Fixed #13452 - Document that regroup can be used with any template...

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

Backport of r15108 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15109 bcc190cf-cafb-0310-a4f2-bffc1f526a37