Update wait_for_publish() documentation.

src/paho/mqtt/client.py

@@ -337,7 +337,17 @@ def _set_as_published(self):
  self._condition.notify()
 
  def wait_for_publish(self, timeout=None):
- """Block until the message associated with this object is published."""
+ """Block until the message associated with this object is published, or
+ until the timeout occurs. If timeout is None, this will never time out.
+ Set timeout to a positive number of seconds, e.g. 1.2, to enable the
+ timeout.
+
+ Raises ValueError if the message was not queued due to the outgoing
+ queue being full.
+
+ Raises RuntimeError if the message was not published for another
+ reason.
+ """
  if self.rc == MQTT_ERR_QUEUE_SIZE:
  raise ValueError('Message is not queued due to ERR_QUEUE_SIZE')
  elif self.rc == MQTT_ERR_AGAIN: