Clarified docs for some tags and filters
Fixed #20958 -- Documented that GenericForeignKey fields can't be accessed in forms.

Thanks marky1991.
Fixed #21010 -- Changed ModelState to only copy _meta.local_fields.
Updated six to version 1.4.1.
Tested exc_type instead of exc_value in __exit__.

exc_value might be None even though there's an exception, at least on
Python 2.6. Thanks Thomas Chaumeny for the report.

Fixed #21034.

Forward-port of a8624b2 from 1.6.x.
Simplify FilterExpression.args_check