Fixed #16758 - Added a warning regarding overriding default settings; thanks…

Fixed #16758 - Added a warning regarding overriding default settings; thanks cyclops for the suggestion & Aymeric Augustin for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17566 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/ref/settings.txt

@@ -6,6 +6,13 @@ Settings
     :local:
     :depth: 1
 
+.. warning::
+
+    Be careful when you override settings, especially when the default value
+    is a non-empty tuple or dict, like :setting:`MIDDLEWARE_CLASSES` and
+    :setting:`TEMPLATE_CONTEXT_PROCESSORS`. Make sure you keep the components
+    required by the features of Django you wish to use.
+
 Available settings
 ==================