Add StaticRNN.output code example (#18251)

refine StaticRNN api doc
test=develop
test=document_preview

python/paddle/fluid/layers/control_flow.py

@@ -307,6 +307,7 @@ class StaticRNN(object):
                 hidden = fluid.layers.fc(input=[word, prev], size=hidden_size, act='relu')
                 rnn.update_memory(prev, hidden)  # set prev to hidden
                 rnn.step_output(hidden)
+                rnn.output(word)
 
             result = rnn()