Explicitly document delete-on-failure requirement.

tseaver · tseaver · commit 8b0fe9785fe5 · 2016-07-14T10:50:47.000-04:00
Addresses: googleapis#1636 (comment) googleapis#1636 (comment)
diff --git a/gcloud/pubsub/subscription.py b/gcloud/pubsub/subscription.py
@@ -446,7 +446,9 @@ class AutoAck(dict):
     Mapping, tracks messages still-to-be-acknowledged.
 
     When used as a context manager, acknowledges all messages still in the
-    mapping on `__exit__`.  E.g.:
+    mapping on `__exit__`.  When processing the pulled messsages, application
+    code MUST delete messages from the :class:`AutoAck` mapping which are not
+    successfully processed, e.g.:
 
     .. code-block: python
 
