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

docs/topics/settings.txt

@@ -220,7 +220,7 @@ In this example, default settings are taken from ``myapp_defaults``, and the
 The following example, which uses ``myapp_defaults`` as a positional argument,
 is equivalent::
 
-    settings.configure(myapp_defaults, DEBUG = True)
+    settings.configure(myapp_defaults, DEBUG=True)
 
 Normally, you will not need to override the defaults in this fashion. The
 Django defaults are sufficiently tame that you can safely use them. Be aware
@@ -242,7 +242,16 @@ is accessed.
 
 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.