message: Extended GRU.call() documentation
  As requested here:
diff: @@ -237,13 +237,14 @@ class GRU(recurrent.DropoutRNNCellMixin, recurrent.GRU):
  Call arguments:
  inputs: A 3D tensor.
  mask: Binary tensor of shape `(samples, timesteps)` indicating whether
  - a given timestep should be masked.
  + a given timestep should be masked (optional, defaults to `None`).
  training: Python boolean indicating whether the layer should behave in
  training mode or in inference mode. This argument is passed to the cell
  when calling it. This is only relevant if `dropout` or
  - `recurrent_dropout` is used.
  + `recurrent_dropout` is used (optional, defaults to `None`).
  initial_state: List of initial state tensors to be passed to the first
  - call of the cell.
  + call of the cell (optional, defaults to `None` which causes creation
  + of zero-filled initial state tensors).
  """
  def __init__(self,
  