!3487 Modify comment of register_backward_hook

Merge pull request !3487 from Simson/push-to-opensource

mindspore/nn/cell.py

@@ -862,7 +862,7 @@ class Cell:
 
     def register_backward_hook(self, fn):
         """
-        Set the cell backward hook function.
+        Set the cell backward hook function. Note that this function is only supported in Pynative Mode.
 
         Note:
             fn should be defined as following code shows, `cell_name` is the name of registered cell,

mindspore/ops/operations/debug_ops.py

@@ -251,7 +251,8 @@ class InsertGradientOf(PrimitiveWithInfer):
 
 class HookBackward(PrimitiveWithInfer):
     """
-    Used as tag to hook gradient in intermediate variables.
+    Used as tag to hook gradient in intermediate variables.  Note that this function
+    is only supported in Pynative Mode.
 
     Note:
         The hook function should be defined like `hook_fn(grad) -> Tensor or None`,