polish in_dygraph_mode (#20200)

* polish in_dygraph_mode, test=document_fix

* test=document_fix, test=develop

* test=document_fix, test=develop

paddle/fluid/API.spec

@@ -14,7 +14,7 @@ paddle.fluid.name_scope (ArgSpec(args=['prefix'], varargs=None, keywords=None, d
 paddle.fluid.cuda_places (ArgSpec(args=['device_ids'], varargs=None, keywords=None, defaults=(None,)), ('document', '1f2bb6ece651e44117652d2d7bedecf5'))
 paddle.fluid.cpu_places (ArgSpec(args=['device_count'], varargs=None, keywords=None, defaults=(None,)), ('document', '956bab564ebc69ffd17195c08cc8ffa0'))
 paddle.fluid.cuda_pinned_places (ArgSpec(args=['device_count'], varargs=None, keywords=None, defaults=(None,)), ('document', 'c2562241744aabe3fff1b59af22dd281'))
-paddle.fluid.in_dygraph_mode (ArgSpec(args=[], varargs=None, keywords=None, defaults=None), ('document', '301bae0d8e02cc9eec5be02f052f11c6'))
+paddle.fluid.in_dygraph_mode (ArgSpec(args=[], varargs=None, keywords=None, defaults=None), ('document', 'df1f4d1ed7e1eefe04f6361efda6b75a'))
 paddle.fluid.is_compiled_with_cuda (ArgSpec(args=[], varargs=None, keywords=None, defaults=None), ('document', '60c7f107a5050aeb58bb74eb175672b5'))
 paddle.fluid.Variable ('paddle.fluid.framework.Variable', ('document', '65ff735c2b96673d7131f5ff6b0db40c'))
 paddle.fluid.Variable.__init__ (ArgSpec(args=['self', 'block', 'type', 'name', 'shape', 'dtype', 'lod_level', 'capacity', 'persistable', 'error_clip', 'stop_gradient', 'is_data', 'need_check_feed'], varargs=None, keywords='kwargs', defaults=(VarType.LOD_TENSOR, None, None, None, None, None, None, None, False, False, False)), ('document', '6adf97f83acf6453d4a6a4b1070f3754'))

python/paddle/fluid/framework.py

@@ -62,17 +62,20 @@ _dygraph_current_expected_place_ = None
 
 def in_dygraph_mode():
     """
-    Check program status(tracer), Whether it runs in dygraph mode or not
+    This function checks whether the program runs in dynamic graph mode or not.
+    You can turn on dynamic graph mode with :ref:`api_fluid_dygraph_guard` api.
 
     Returns:
-        out (boolean): True if the program is running in dynamic graph mode
+        bool: Whether the program is running in dynamic graph mode.
 
     Examples:
         .. code-block:: python
 
             import paddle.fluid as fluid
             if fluid.in_dygraph_mode():
-                pass
+                print('running in dygraph mode')
+            else:
+                print('not running in dygraph mode')
 
     """
     return _dygraph_tracer_ is not None