docs: add flow control section to publish overview (#129)

* docs: add flow control section to publish overview

* Explain options for limit exceeded behavior

* Omit a sentence that might cause confusion.

docs/publisher/index.rst

@@ -128,6 +128,42 @@ You can also attach a callback to the future:
     future.add_done_callback(callback)
 
 
+Publish Flow Control
+--------------------
+
+If publishing large amounts of messages or very large messages in quick
+succession, some of the publish requests might time out, especially if the
+bandwidth available is limited. To mitigate this the client can be
+configured with custom :class:`~.pubsub_v1.types.PublishFlowControl` settings.
+
+You can configure the maximum desired number of messages and their maximum total
+size, as well as the action that should be taken when the threshold is reached.
+
+.. code-block:: python
+
+    from google.cloud import pubsub_v1
+
+    client = pubsub_v1.PublisherClient(
+        publisher_options=pubsub_v1.types.PublisherOptions(
+            flow_control=pubsub_v1.types.PublishFlowControl(
+                message_limit=500,
+                byte_limit=2 * 1024 * 1024,
+                limit_exceeded_behavior=pubsub_v1.types.LimitExceededBehavior.BLOCK,
+            ),
+        ),
+    )
+
+The action to be taken on overflow can be one of the following:
+
+* :attr:`~.pubsub_v1.types.LimitExceededBehavior.IGNORE` (default): Ignore the
+  overflow and continue publishing the messages as normal.
+* :attr:`~.pubsub_v1.types.LimitExceededBehavior.ERROR`: Raise
+  :exc:`~.pubsub_v1.publisher.exceptions.FlowControlLimitError` and reject the message.
+* :attr:`~.pubsub_v1.types.LimitExceededBehavior.BLOCK`: Temporarily block in the
+  :meth:`~.pubsub_v1.publisher.client.Client.publish` method until there is
+  enough capacity available.
+
+
 API Reference
 -------------