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

Added correct information for how to enable rnn statefulness on functional models with 1 or more Input layers.
DeepGnosis/keras,relh/keras,kuza55/keras,keras-team/keras,nebw/keras,kemaswill/keras,keras-team/keras,dolaameng/keras