Clarified the fact that the signal_changed signal isn't used by Django itself (yet). Refs #17787.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17597 bcc190cf-cafb-0310-a4f2-bffc1f526a37

docs/topics/testing.txt

@@ -1477,11 +1477,13 @@ decorate the class::
 
 .. note::
 
-    When overriding settings make sure to also handle the cases in which
-    Django or your app's code uses a cache or similar feature that retains
-    state even if the setting is changed. Django provides the
-    :data:`django.test.signals.setting_changed` signal to connect cleanup
-    and other state-resetting callbacks to.
+    When overriding settings make sure to also handle the cases in which your
+    app's code uses a cache or similar feature that retains state even if the
+    setting is changed. Django provides the
+    :data:`django.test.signals.setting_changed` signal to connect cleanup and
+    other state-resetting callbacks to. Note that this signal isn't currently
+    used by Django itself, so changing built-in settings may not yield the
+    results you expect.
 
 Emptying the test outbox
 ~~~~~~~~~~~~~~~~~~~~~~~~