Fix issue #18267 - document `settings.configured` property.

If you set ``DJANGO_SETTINGS_MODULE``, access settings values somehow, *then*

call ``configure()``, Django will raise a ``RuntimeError`` indicating

that settings have already been configured.

that settings have already been configured. There is a property just for this

purpose:

.. attribute: django.conf.settings.configured

For example::

    from django.conf import settings

    if not settings.configured:

        settings.configure(myapp_defaults, DEBUG=True)

Also, it's an error to call ``configure()`` more than once, or to call

``configure()`` after any setting has been accessed.