[1.2.X] Fixed #5537 -- document trailing '+' on related_name for supressing backward relation.

Thanks to dcramer for the report, and Russ for pointing out the workaround.

Backport of r14049 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14050 bcc190cf-cafb-0310-a4f2-bffc1f526a37