message: Update description of TPUEstimator.
diff: @@ -1724,6 +1724,9 @@ class InstallSignalHandlerHook(session_run_hook.SessionRunHook):
  class TPUEstimator(estimator_lib.Estimator):
  """Estimator with TPU support.
  + TPUEstimator also supports training on CPU and GPU. You don't need to define
  + a separate `tf.estimator.Estimator`.
  +
  TPUEstimator handles many of the details of running on TPU devices, such as
  replicating inputs and models for each core, and returning to host
  periodically to run hooks.
  