Fixed #12412 -- Clarified the documentation around file handling by the test...

Fixed #12412 -- Clarified the documentation around file handling by the test client. Thanks to Gabriel Hurley for the draft patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13166 bcc190cf-cafb-0310-a4f2-bffc1f526a37