message: add example in docstring
diff: @@ -2530,6 +2530,19 @@ name=None))
  You may provide either a constant `window_size` or a window size determined
  by the key through `window_size_func`.
  + >>> dataset = tf.data.Dataset.range(10)
  + >>> window_size = 5
  + >>> key_func = lambda x: x%2
  + >>> reduce_func = reduce_func=lambda key, dataset: dataset.batch(window_size)
  + >>> dataset = dataset.group_by_window(
  + ... key_func=key_func,
  + ... reduce_func=reduce_func,
  + ... window_size=window_size)
  + >>> for elem in dataset.as_numpy_iterator():
  + ... print(elem)
  + [0 2 4 6 8]
  + [1 3 5 7 9]
  +
  Args:
  key_func: A function mapping a nested structure of tensors
  (having shapes and types defined by `self.output_shapes` and
  