message: explain what SRU stands for
diff: @@ -2634,7 +2634,10 @@ class LayerNormLSTMCell(rnn_cell_impl.RNNCell):
  class SRUCell(rnn_cell_impl._LayerRNNCell):
  - """Training RNNs as Fast as CNNs (cf. https://arxiv.org/abs/1709.02755).
  + """SRU, Simple Recurrent Unit
  + Implementation based on
  + Training RNNs as Fast as CNNs (cf. https://arxiv.org/abs/1709.02755).
  +
  This variation of RNN cell is characterized by the simplified data dependence
  between hidden states of two consecutive time steps. Traditionally, hidden
  states from a cell at time step t-1 needs to be multiplied with a matrix
  