message: add example for new scan api usage
diff: @@ -2683,6 +2683,13 @@ name=None))
  `scan()` accumulates one or more state tensors, whose initial values are
  `initial_state`.
  + >>> dataset = tf.data.Dataset.range(10)
  + >>> initial_state = tf.constant(0, dtype=tf.int64)
  + >>> scan_func = lambda state, i: (state + i, state + i)
  + >>> dataset = dataset.scan(initial_state=initial_state, scan_func=scan_func)
  + >>> list(dataset.as_numpy_iterator())
  + [0, 1, 3, 6, 10, 15, 21, 28, 36, 45]
  +
  Args:
  initial_state: A nested structure of tensors, representing the initial
  state of the accumulator.
  