update paramattr doc. test=document_fix (#28745)

python/paddle/fluid/param_attr.py

@@ -56,7 +56,7 @@ class ParamAttr(object):
                 no regularization.
         trainable (bool): Whether this parameter is trainable. Default True.
         do_model_average (bool): Whether this parameter should do model average
-                when model average is enabled. Default False.
+                when model average is enabled. Only used in ExponentialMovingAverage. Default True.
         need_clip (bool): Whether the parameter gradient need to be cliped in optimizer. Default is True.
 
     Examples: