Added to rnn statefulness doc concerning func api (keras-team#3375)

Added correct information for how to enable rnn statefulness on functional models with 1 or more Input layers.

keras/layers/recurrent.py

@@ -139,7 +139,10 @@ class Recurrent(Layer):
         To enable statefulness:
             - specify `stateful=True` in the layer constructor.
             - specify a fixed batch size for your model, by passing
-                a `batch_input_shape=(...)` to the first layer in your model.
+                if sequential model:
+                  a `batch_input_shape=(...)` to the first layer in your model.
+                else for functional model with 1 or more Input layers:
+                  a `batch_shape=(...)` to all the first layers in your model.
                 This is the expected shape of your inputs *including the batch size*.
                 It should be a tuple of integers, e.g. `(32, 10, 100)`.