[1.3.X] Fixed #15876 - Document that test.client.RequestFactory doesn't...

[1.3.X] Fixed #15876 - Document that test.client.RequestFactory doesn't support sessions or request-altering middleware; thanks slinkp for the suggestion, ShawnMilo for the patch.

Backport of r16128 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16129 bcc190cf-cafb-0310-a4f2-bffc1f526a37