message: Improve tf.identity documentation
diff: @@ -272,11 +272,12 @@ def identity(input, name=None): # pylint: disable=redefined-builtin
  5
  Args:
  - input: A `Tensor`.
  + input: A `Tensor`, a `Variable`, a `CompositeTensor` or anything that can be
  + converted to a tensor using `tf.convert_to_tensor`.
  name: A name for the operation (optional).
  Returns:
  - A `Tensor`. Has the same type as `input`.
  + A `Tensor` or CompositeTensor. Has the same type and contents as `input`.
  """
  if isinstance(input, composite_tensor.CompositeTensor):
  return nest.map_structure(identity, input, expand_composites=True)
  