[1.7.x] Fixed #22924 -- Added a note about MEDIA_URL in templates.

Thanks raphael.herouart at gmail.com

Backport of 814bcc33 from master

docs/ref/settings.txt

@@ -1728,6 +1728,11 @@ for :doc:`managing stored files </topics/files>`. It must end in a slash if set
 to a non-empty value. You will need to :ref:`configure these files to be served
 <serving-uploaded-files-in-development>` in both development and production.
 
+In order to use ``{{ MEDIA_URL }}`` in your templates, you must have
+``'django.core.context_processors.media'`` in your
+:setting:`TEMPLATE_CONTEXT_PROCESSORS`. It's there by default, but be sure
+to include it if you override that setting and want this behavior.
+
 Example: ``"http://media.example.com/"``
 
 .. warning::