Pub/Sub: document regional endpoint (#8789)

* document client_options

* show an example that sets client_options

Author: anguillanneuf

google/cloud/pubsub_v1/publisher/client.py

@@ -58,6 +58,9 @@ class Client(object):
             Before being passed along to the GAPIC constructor, a channel may
             be added if ``credentials`` are passed explicitly or if the
             Pub / Sub emulator is detected as running.
+            Regional endpoints can be set via `client_options` that takes a
+            single key-value pair that defines the endpoint, i.e.
+            `client_options={"api_endpoint": REGIONAL_ENDPOINT}`
     """
 
     _batch_class = thread.Batch

google/cloud/pubsub_v1/subscriber/client.py

@@ -53,6 +53,9 @@ class Client(object):
             :class:`~.gapic.pubsub.v1.subscriber_client.SubscriberClient`.
             Generally, you should not need to set additional keyword
             arguments.
+            Regional endpoints can be set via `client_options` that takes a
+            single key-value pair that defines the endpoint, i.e.
+            `client_options={"api_endpoint": REGIONAL_ENDPOINT}`.
     """
 
     def __init__(self, **kwargs):