message: Update RNN document for the `cell` parameter type.
diff: @@ -235,7 +235,8 @@ class RNN(Layer):
  """Base class for recurrent layers.
  Arguments:
  - cell: A RNN cell instance. A RNN cell is a class that has:
  + cell: A RNN cell instance or a list of RNN cell instances.
  + A RNN cell is a class that has:
  - a `call(input_at_t, states_at_t)` method, returning
  `(output_at_t, states_at_t_plus_1)`. The call method of the
  cell can also take the optional argument `constants`, see
  @@ -248,9 +249,9 @@ class RNN(Layer):
  (one size per state). In this case, the first entry
  (`state_size[0]`) should be the same as
  the size of the cell output.
  - It is also possible for `cell` to be a list of RNN cell instances,
  - in which cases the cells get stacked on after the other in the RNN,
  - implementing an efficient stacked RNN.
  + In the case that `cell` is a list of RNN cell instances, the cells
  + will be stacked on after the other in the RNN, implementing an
  + efficient stacked RNN.
  return_sequences: Boolean. Whether to return the last output
  in the output sequence, or the full sequence.
  return_state: Boolean. Whether to return the last state
  