Refine English Doc of cond API (#21609)

As the title

python/paddle/fluid/layers/control_flow.py

@@ -1851,24 +1851,45 @@ def cond(pred, true_fn=None, false_fn=None, name=None):
     list of tensors.
     
     Note: 
-        The tuples or lists in ``true_fn`` and ``false_fn`` must have same
+        1. The tuples or lists returned by ``true_fn`` and ``false_fn`` must have
-        shape because of dataflow model of PaddlePaddle while the tensors in the
+        the same shape because of dataflow model of PaddlePaddle while the
-        tuples or the lists can have different shapes.
+        tensors in the tuples or the lists can have different shapes.
+
+        2. Any tensors or operations created outside of ``true_fn`` and
+        ``false_fn`` will be executed regardless of which branch is selected at
+        runtime. This has frequently surprised users who expected a lazy
+        semantics. For example:
+
+        .. code-block:: python
+        
+            import paddle.fluid as fluid
+            a = fluid.data(name='a', shape=[-1, 1], dtype='float32')
+            b = fluid.data(name='b', shape=[-1, 1], dtype='float32')
+            c = a * b
+            out = fluid.layers.cond(a < b, lambda: a + c, lambda: b * b)
+
+        No matter whether ``a < b`` , ``c = a * b`` will run.
 
     Args:
         pred(Variable): A boolean tensor whose numel should be 1. The boolean
             value determines whether to return the result of ``true_fn`` or
-            ``false_fn``
+            ``false_fn`` .
-        true_fn(callable): A callable to be performed if ``pred`` is true
+        true_fn(callable, optional): A callable to be performed if ``pred`` is
-        false_fn(callable): A callable to be performed if ``pred`` is false
+            true. The default value is ``None`` .
+        false_fn(callable, optional): A callable to be performed if ``pred`` is
+            false. The default value is ``None`` .
         name(str, optional): The default value is ``None`` . Normally users
              don't have to set this parameter. For more information, please
              refer to :ref:`api_guide_Name` .
 
+    Returns:
+        Variable|list(Variable)|tuple(Variable): returns ``true_fn()`` if the
+        predicate ``pred`` is true else ``false_fn()`` .
+
     Raises:
         TypeError: if ``true_fn`` or ``false_fn`` is not callable.
-        ValueError: if ``true_fn`` and ``false_fn`` doesn't return the same
+        ValueError: if ``true_fn`` and ``false_fn`` don't return the same nest
-            nest structure of tensors.
+            structure of tensors.
 
     Examples:
         .. code-block:: python