[1.1.X] Fixed #12114 -- Clarified the existence of the reverse relation _set...

[1.1.X] Fixed #12114 -- Clarified the existence of the reverse relation _set in the tutorial. Thanks to panfist for the suggestion, and dwillis for the patch.

Backport of r12711 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12714 bcc190cf-cafb-0310-a4f2-bffc1f526a37