Fixed #25804 -- Documented additions to Jinja2 context.

* ``'auto_reload'``: ``settings.DEBUG``

* ``'undefined'``: ``DebugUndefined if settings.DEBUG else Undefined``

The default configuration is purposefully kept to a minimum. The ``Jinja2``

backend doesn't create a Django-flavored environment. It doesn't know about

Django context processors, filters, and tags. In order to use Django-specific

APIs, you must configure them into the environment.

The default configuration is purposefully kept to a minimum. If a template is

rendered with a request (e.g. when using :py:func:`~django.shortcuts.render`),

the ``Jinja2`` backend adds the globals ``request``, ``csrf_input``, and

``csrf_token`` to the context. Apart from that, this backend doesn't create a

Django-flavored environment. It doesn't know about Django context processors,

filters, and tags. In order to use Django-specific APIs, you must configure

them into the environment.

For example, you can create ``myproject/jinja2.py`` with this content::