message: [tf.data] Update Dataset.prefetch doc to mention the autotune option.
diff: @@ -1159,8 +1159,8 @@ class DatasetV2(collections_abc.Iterable, tracking_base.Trackable,
  Args:
  buffer_size: A `tf.int64` scalar `tf.Tensor`, representing the maximum
  - number of elements that will be buffered when prefetching.
  -
  + number of elements that will be buffered when prefetching. If the value
  + `tf.data.AUTOTUNE` is used, then the buffer size is dynamically tuned.
  Returns:
  Dataset: A `Dataset`.
  """
  