add comment for revise, test=develop

python/paddle/fluid/layers/nn.py

@@ -1796,7 +1796,8 @@ def softmax(input, use_cudnn=False, name=None):
     Args:
         input (Variable): The input variable.
         use_cudnn (bool): Use cudnn kernel or not, it is valid only when the cudnn \
-            library is installed.
+            library is installed. To improve numerical stablity, set use_cudnn to \
+            False by default. Default: False
         name (str|None): A name for this layer(optional). If set None, the layer
             will be named automatically. Default: None.