task-cookbook: Make warning more prominent

docs/tutorials/task-cookbook.rst

@@ -29,6 +29,12 @@ and something always will...
 For this reason your tasks run-time should not exceed the timeout.
 
 
+.. note::
+
+    In order for this to work correctly you need to be using a cache
+    backend where the ``.add`` operation is atomic.  ``memcached`` is known
+    to work well for this purpose.
+
 .. code-block:: python
 
     from celery import task
@@ -71,8 +77,3 @@ For this reason your tasks run-time should not exceed the timeout.
                 return Feed.objects.import_feed(feed_url).url
         logger.debug(
             'Feed %s is already being imported by another worker', feed_url)
-
-
-Note that in order for this to work correctly you need to be using a cache
-backend that supports an atomic ``.add`` operation.  ``memcached`` is known
-to work well for this purpose.