Fixed #11711 -- clarified that ValuesQuerySets are not lists per se.

It rarely hurts to think of the returned result from a values() or
values_list() call as a list, but it's really an iterable and sometimes
the difference matters.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12743 bcc190cf-cafb-0310-a4f2-bffc1f526a37