[1.1.X] Fixed #12412 -- Clarified the documentation around file handling by...

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

Backport of r13166 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@13172 bcc190cf-cafb-0310-a4f2-bffc1f526a37