docs: document potentially unexpected blocking behavior of publish() …

…method (#214)

* docs: Document potentially unexpected blocking behavior of publish() method.

* Update google/cloud/pubsub_v1/publisher/client.py

Co-authored-by: Tres Seaver <tseaver@palladion.com>
Co-authored-by: Prad Nelluru <pradn@google.com>

google/cloud/pubsub_v1/publisher/client.py

@@ -291,6 +291,8 @@ def publish(
         Add the given message to this object; this will cause it to be
         published once the batch either has enough messages or a sufficient
         period of time has elapsed.
+        This method may block if LimitExceededBehavior.BLOCK is used in the
+        flow control settings.
 
         Example:
             >>> from google.cloud import pubsub_v1