!13828 Add notes for parameter 'flags' of cell.

From: @wang_shaocong
Reviewed-by: @gemini524,@c_34,@liangchenghui
Signed-off-by: @c_34

mindspore/nn/cell.py

@@ -55,6 +55,8 @@ class Cell(Cell_):
 
     Args:
         auto_prefix (bool): Recursively generate namespaces. Default: True.
+        flags (dict): Network configuration information, currently it is used for the binding of network and dataset.
+                      Users can also customize network attributes by this parameter. Default: None.
 
     Supported Platforms:
         ``Ascend`` ``GPU`` ``CPU``