add Note in the doc of old control flow ops. test=develop,test=document_fix (#22108)

python/paddle/fluid/layers/control_flow.py

@@ -829,6 +829,10 @@ class While(object):
     """
     while loop control flow. Repeat while body until cond is False.
 
+    Note:
+        A new OP :ref:`api_fluid_layers_while_loop` is highly recommended instead of ``While`` if the shape of parameter ``cond`` is [1].
+        OP :ref:`api_fluid_layers_while_loop` is easier to use and is called with less code but does the same thing as ``While`` .
+
     Args:
         cond(Variable): A Tensor whose data type is bool controlling whether to continue looping.
         is_test(bool, optional): A flag indicating whether execution is in test phase. Default value is False.
@@ -2210,6 +2214,10 @@ class Switch(object):
     If there is no case branch that satisfies the condition, 
     only the statement following the default branch is executed.
 
+    Note:
+        A new OP :ref:`api_fluid_layers_case` is highly recommended instead of ``Switch`` if the shape of parameter ``cond`` is [1].
+        OP :ref:`api_fluid_layers_case` is easier to use and is called with less code but does the same thing as ``Switch`` .
+
     Member Functions:
         case(cond): The case branch of Switch whose parameter cond is a scalar Variable of bool type. Only if the cond of the current case branch is True and the cond of the previous case branch is False, the statement after the case branch will be executed, and the statement after the case branch will not be executed.
         
@@ -2355,6 +2363,10 @@ class IfElse(object):
 
     Cond is a 2-D Tensor with shape [N, 1] and data type bool, representing the execution conditions of the corresponding part of the input data.
 
+    Note:
+        A new OP :ref:`api_fluid_layers_cond` is highly recommended instead of ``IfElse``. if the shape of parameter ``cond`` is [1].
+        OP :ref:`api_fluid_layers_cond` is easier to use and is called with less code but does the same thing as ``IfElse`` .
+
     IfElse OP is different from other OPs in usage, which may cause some users confusion. Here is a simple example to illustrate this OP.
 
     .. code-block:: python