message: [tf.data] Add note about performance cost of `Dataset.unbatch`.
diff: @@ -2099,6 +2099,10 @@ name=None))
  >>> list(dataset.as_numpy_iterator())
  [1, 2, 3, 1, 2, 1, 2, 3, 4]
  + Note: `unbatch` requires a data copy to slice up the batched tensor into
  + smaller, unbatched tensors. When optimizing performance, try to avoid
  + unnecessary usage of `unbatch`.
  +
  Returns:
  A `Dataset`.
  """
  