Add docstring for get_notification_transport

This was previously undocumented.  Most of the information was taken from the
docstring for get_transport

Change-Id: I92652f4de9dd1e59ae5c3f9ce39e5ee808f24c7d

oslo_messaging/notify/notifier.py

@@ -138,6 +138,31 @@ class Driver(object):
 
 def get_notification_transport(conf, url=None,
                                allowed_remote_exmods=None, aliases=None):
+    """A factory method for Transport objects for notifications.
+
+    This method should be used for notifications, in case notifications are
+    being sent over a different message bus than normal messaging
+    functionality; for example, using a different driver, or with different
+    access permissions.
+
+    If no transport URL is provided, the URL in the notifications section of
+    the config file will be used.  If that URL is also absent, the same
+    transport as specified in the messaging section will be used.
+
+    If a transport URL is provided, then this function works exactly the same
+    as get_transport.
+
+    :param conf: the user configuration
+    :type conf: cfg.ConfigOpts
+    :param url: a transport URL
+    :type url: str or TransportURL
+    :param allowed_remote_exmods: a list of modules which a client using this
+                                  transport will deserialize remote exceptions
+                                  from
+    :type allowed_remote_exmods: list
+    :param aliases: A map of transport alias to transport name
+    :type aliases: dict
+    """
     conf.register_opts(_notifier_opts,
                        group='oslo_messaging_notifications')
     if url is None: