message: Add an explanatory paragraph to `shuffle`.
diff: @@ -750,6 +750,12 @@ class DatasetV2(object):
  elements. For perfect shuffling, a buffer size greater than or equal to the
  full size of the dataset is required.
  + For instance, if your dataset contains 10,000 elements but `buffer_size` is
  + set to 1,000, then `shuffle` will initially select a random element from
  + only the first 1,000 elements in the buffer. Once an element is selected,
  + its space in the buffer is replaced by the next (i.e. 1,001-st) element,
  + maintaining the 1,000 element buffer.
  +
  Args:
  buffer_size: A `tf.int64` scalar `tf.Tensor`, representing the
  number of elements from this dataset from which the new
  