Ensure event time listener is in listeners docs

Also expands the docstring so its known what keys
this stores under and what data it is recording.

Change-Id: Id2949ca6acc27d4bf41e37743f705d653f426c05

doc/source/notifications.rst

@@ -170,6 +170,8 @@ Timing listeners
 
 .. autoclass:: taskflow.listeners.timing.PrintingDurationListener
 
+.. autoclass:: taskflow.listeners.timing.EventTimeListener
+
 Claim listener
 --------------
 
@@ -191,6 +193,7 @@ Hierarchy
     taskflow.listeners.logging.DynamicLoggingListener
     taskflow.listeners.logging.LoggingListener
     taskflow.listeners.printing.PrintingListener
-    taskflow.listeners.timing.PrintingTimingListener
-    taskflow.listeners.timing.TimingListener
+    taskflow.listeners.timing.PrintingDurationListener
+    taskflow.listeners.timing.EventTimeListener
+    taskflow.listeners.timing.DurationListener
     :parts: 1

taskflow/listeners/timing.py

@@ -121,7 +121,16 @@ PrintingTimingListener = moves.moved_class(
 
 
 class EventTimeListener(base.Listener):
-    """Writes task, flow, and retry event timestamps to atom metadata."""
+    """Listener that captures task, flow, and retry event timestamps.
+
+    It records how when an event is received (using unix time) to
+    storage. It saves the timestamps under keys (in atom or flow details
+    metadata) of the format ``{event}-timestamp`` where ``event`` is the
+    state/event name that has been received.
+
+    This information can be later extracted/examined to derive durations...
+    """
+
     def __init__(self, engine,
                  task_listen_for=base.DEFAULT_LISTEN_FOR,
                  flow_listen_for=base.DEFAULT_LISTEN_FOR,