Clearly explain how to use memory cache backend

Admittedly I am slow, but it shouldn't have taken me 10 minutes to figure out how to set up cache/memory (very useful for `unittest`s).

docs/configuration.rst

@@ -396,6 +396,9 @@ Using multiple memcached servers:
 
 The "memory" backend stores the cache in memory only:
 
+.. code-block:: python
+
+    CELERY_RESULT_BACKEND = 'cache'
     CELERY_CACHE_BACKEND = 'memory'
 
 CELERY_CACHE_BACKEND_OPTIONS