add docstring

python/paddle/v2/fluid/clip.py

@@ -160,6 +160,17 @@ class GradientClipByGlobalNorm(BaseGradientClipAttr):
 
 
 def set_gradient_clip(clip, param_list=None, program=None):
+    """
+        To specify parameters that require gradient clip.
+        Args:
+            clip(BaseGradientClipAttr): An instance of some derived class of BaseGradientClipAttr, 
+                    which describes the type and detailed attributes of required gradient clip.
+            param_list(list, None by default): Parameters that require gradient clip. 
+                    It can be a list of parameter or a list of parameter's name. 
+                    When it's None, all parameters in the program will be included. 
+            program(Program, None by default): The program where parameters are. 
+                    Will be the default main program when assigned with None.
+    """
     if not isinstance(clip, BaseGradientClipAttr):
         raise TypeError(
             "'clip' should be an instance of BaseGradientClipAttr's derived class"