message: Add a warning against calling _tf_sess() in user code.
diff: @@ -840,6 +840,14 @@ class _MonitoredSession(object):
  return self._coordinated_creator.tf_sess is None
  def _tf_sess(self):
  + """Return underlying tf.Session object.
  +
  + Warning: accessing the returned object in user code is likely to cause races
  + or "flaky tests".
  +
  + Returns:
  + A tf.Session object.
  + """
  return self._coordinated_creator.tf_sess
  