message: add clear user instructions for dataset.prefetch
diff: @@ -699,8 +699,8 @@ class DatasetV2(tracking_base.Trackable, composite_tensor.CompositeTensor):
  def prefetch(self, buffer_size):
  """Creates a `Dataset` that prefetches elements from this dataset.
  - Note that if the dataset was batched using `Dataset.batch`, each element is
  - a batch and this operation will prefetch `buffer_size` batches.
  + NOTE: If prefetch after batch, such as: dataset.batch(20).prefetch(2),
  + which means prefetch 2 batches, totally 40 samples.
  Args:
  buffer_size: A `tf.int64` scalar `tf.Tensor`, representing the maximum
  