Add comment to the PythonAPI

Author: gaoyuan

python/paddle/trainer_config_helpers/layers.py

@@ -1223,7 +1223,8 @@ def detection_output_layer(input_loc,
                            name=None):
     """
     Apply the NMS to the output of network and compute the predict bounding
-    box location.
+    box location. The output of this layer could be None if there is no valid
+    bounding box.
 
     :param name: The Layer Name.
     :type name: basestring