Merge pull request #3047 from tseaver/3037-pubsub-clarify_reload_semantics

Expand motivation in 'Subscription.reload' docstring.

Author: tseaver

pubsub/google/cloud/pubsub/subscription.py

@@ -220,6 +220,10 @@ def reload(self, client=None):
         See
         https://cloud.google.com/pubsub/docs/reference/rest/v1/projects.subscriptions/get
 
+        :attr:`ack_deadline` and :attr:`push_endpoint` might never have
+        been set locally, or might have been updated by another client.  This
+        method fetches their values from the server.
+
         Example:
 
         .. literalinclude:: pubsub_snippets.py