Document the settings affecting csrf cookie

[wiml]

I think it would be helpful to describe the settings which affect the CSRF cookie in the csrf documentation.

Here is some text which could be appended to docs/ref/contrib/csrf.txt (modeled after the similar section in the session documentation):

Settings
========

A few :ref:`Django settings <ref-settings>` give you control over the CSRF cookie:

CSRF_COOKIE_NAME
----------------

Default: ``csrftoken``

The name of the cookie to use for CSRF tokens.

CSRF_COOKIE_DOMAIN
------------------

Default: ``None``

The domain to use for the CSRF cookie. See :meth:`HttpResponse.set_cookie()`.

[Gabriel Hurley]

These settings are documented: ​http://docs.djangoproject.com/en/dev/ref/settings/#std:setting-CSRF_COOKIE_DOMAIN

Whether the settings for contrib apps should be documented solely in the settings documentation, solely in their respective ref/contrib sections, or in some combination of both is the subject of #14633. If you care to give an opinion on the subject, a comment on #14633 is the place to do it.

Thanks for the report, though!