message: add note on dataset size for unique transformation
diff: @@ -2734,8 +2734,8 @@ name=None))
  >>> sorted(list(dataset.as_numpy_iterator()))
  [1, 2, 37]
  - NOTE: This transformation is only applicable on datasets with either
  - `tf.int32`, `tf.int64` or `tf.string` type elements.
  + NOTE: This transformation only supports datasets which fit into memory
  + and have elements of either `tf.int32`, `tf.int64` or `tf.string` type.
  Returns:
  A `Dataset`.
  