[1.6.x] Clarified docs for some tags and filters

Backport of 1ccdc08189 from master
[1.6.x] Fixed #20958 -- Documented that GenericForeignKey fields can't be accessed in forms.

Thanks marky1991.

Backport of 533d1ab334 from master
Updated six to version 1.4.1.
[1.6.x] 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.
[1.6.x] Fixed #21041 -- Removed a duplicate form in tests.

Thanks tuxcanfly.

Backport of bab039d74c from master
[1.6.x] Fixed #20900 -- Documented RemoteUserBackend.authenticate

Backport of 7b62b80693 from master
[1.6.x] Fixed #21044 -- Documented django.core.urlresolvers.Resolver404

Thanks Keryn Knight for the suggestion.

Backport of eacf060e01 from master
[1.6.x] Fixed #21047 -- Added CLA mesage on the new contributor advice doc

Backport of 93dd31cadf from master
[1.6.x] Fixed #21035 -- Changed docs to treat the acronym SQL phonetically.

The documentation and comments now all use 'an' to
refer to the word SQL and not 'a'.

Backport of 4d13cc56de from master
[1.6.x] Fixed instructions for running a subset of tests.

Backport of cd4068f359 from master
[1.6.x] Fixed #20646 -- Clarified the use of AbstractBaseUser.REQUIRED_FIELDS

Thanks craigbruce.

Backport of db3de52807 from master
[1.6.x] Fixed #19295 -- Documented that CachedStaticFilesStorage isn't compatible with runserver --insecure.

Backport of 028db97503 from master
[1.6.x] Fixed #21049 -- Fixed autoreload for Python 3

Changed th system module values check to return a list.
In Python 3 it returns a dict_view which could occassionally produce
a runtime error of "dictionary changed size during iteration".

Backport of 559cb826b8 from master
[1.6.x] Fixed a link in topics/testing/overview.txt

Backport of 0c295a7718 from master
Merge pull request #1579 from ianawilson/ticket_21058

[1.6.x] Fixed #21058 -- Fixed debug view blowing up when no template is provided to the template rendering functions.

Assistance on this commit from @jambonrose.

Backport of 122020fdb93980df850ae02f61d97da27e2cb515 from master.
[1.6.x] Merge pull request #1580 from ianawilson/ticket_16502

Fixed #16502 -- Fixed a TemplateDoesNotExist error that should be an ImproperlyConfigured.

Assistance on the patch from #jambronrose.

Backport of 9b2dc12b8332389d1bfb9e83123a088a084a6a47 from master.
[1.6.x] Merge pull request #1582 from rca/12756-missing-yaml-module-serializer-error-message

Fixed #12756: Improved error message when yaml module is missing.

Backport of 4f5faa1916e7c8cb72cc9ebf1a1fd964ba6e707b from master.
[1.6.x] Fixed Python 3 syntax error introduced in [c72392da]

Backport of 498014ccd5 from master
Merge pull request #1566 from adamsc64/ticket_11857

Fixed #11857 -- Added missing 'closed' property on TemporaryFile class.

Backport of 926bc42 from trunk.
[1.6.x] Fixed #11811 -- Data-loss bug in queryset.update.

It's now forbidden to call queryset.update(field=instance) when instance
hasn't been saved to the database ie. instance.pk is None.

Conflicts:
	tests/queries/tests.py

Backport of b4cd8169 from master.