[1.1.X] Fixed #13455 -- Clarified the interaction of values() and extra()....

[1.1.X] Fixed #13455 -- Clarified the interaction of values() and extra(). Thanks to Rupe for the report and draft text.

Backport of r13145 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13151 bcc190cf-cafb-0310-a4f2-bffc1f526a37