message: add more ValueError description in dynamic_rnn document
diff: @@ -709,6 +709,10 @@ def _dynamic_rnn_loop(cell,
  Raises:
  ValueError: If the input depth cannot be inferred via shape inference
  from the inputs.
  + ValueError: If time is not the same for all the elements in the
  + input.
  + ValueError: If batch_size is not the same for all the elements
  + in the input.
  """
  state = initial_state
  assert isinstance(parallel_iterations, int), "parallel_iterations must be int"
  