message: DOC: update document
diff: @@ -647,15 +647,17 @@ def assert_element_shape(expected_shapes):
  """Assert the shape of this `Dataset`.
  ```python
  - shapes = [tf.TensorShape([16, 256]), tf.TensorShape(None)]
  + shapes = [tf.TensorShape([16, 256]), tf.TensorShape([None, 2])]
  result = dataset.apply(tf.contrib.data.assert_element_shape(shapes))
  - print(result.output_shapes) # ==> "((16, 256), <unknown>)"
  + print(result.output_shapes) # ==> "((16, 256), (<unknown>, 2))"
  ```
  If dataset shapes and expected_shape, are fully defined, assert they match.
  Otherwise, add assert op that will validate the shapes when tensors are
  evaluated, and set shapes on tensors, respectively.
  + Note that unknown dimension in `expected_shapes` will be ignored.
  +
  Args:
  expected_shapes: A nested structure of `tf.TensorShape` objects.
  