message: Add docs about forcing zero output for mask with Bidirectional
diff: @@ -393,6 +393,9 @@ class Bidirectional(Wrapper):
  Note that the recommended way to create new RNN layers is to write a
  custom RNN cell and use it with `keras.layers.RNN`, instead of
  subclassing `keras.layers.Layer` directly.
  + - When the `returns_sequences` is true, the output of the masked timestep
  + will be zero regardless of the layer's original `zero_output_for_mask`
  + value.
  merge_mode: Mode by which outputs of the forward and backward RNNs will be
  combined. One of {'sum', 'mul', 'concat', 'ave', None}. If None, the
  outputs will not be combined, they will be returned as a list. Default
  