message: Document limitation of using `tf.data.Options` in coordinator.create_per_worker_dataset.
diff: @@ -1201,6 +1201,9 @@ class ClusterCoordinator(object):
  assert remote_value.fetch() == 3
  ```
  + NOTE: A known limitation is `tf.data.Options` is ignored in dataset created
  + by `create_per_worker_dataset`.
  +
  Args:
  dataset_fn: The dataset function that returns a dataset. This is to be
  executed on the workers.
  