docs: explain how to nack a sync pull message (#123)

docs/subscriber/index.rst

@@ -64,9 +64,20 @@ To pull the messages synchronously, use the client's
     subscriber.acknowledge(subscription_path, ack_ids)
 
 The method returns a :class:`~.pubsub_v1.types.PullResponse` instance that
-cointains a list of received :class:`~.pubsub_v1.types.ReceivedMessage`
+contains a list of received :class:`~.pubsub_v1.types.ReceivedMessage`
 instances.
 
+If you want to **nack** some of the received messages (see :ref:`explaining-ack` below),
+you can use the :meth:`~.pubsub_v1.subscriber.client.Client.modify_ack_deadline`
+method and set their acknowledge deadlines to zero. This will cause them to
+be dropped by this client and the backend will try to re-deliver them.
+
+.. code-block:: python
+
+    ack_ids = []  # TODO: populate with `ack_ids` of the messages to NACK
+    ack_deadline_seconds = 0
+    subscriber.modify_ack_deadline(subscription_path, ack_ids, ack_deadline_seconds)
+
 
 Pulling a Subscription Asynchronously
 -------------------------------------
@@ -159,6 +170,8 @@ receiving messages.
     future.cancel()
 
 
+.. _explaining-ack:
+
 Explaining Ack
 --------------