modify doc for ParameterList and LayerList (#27609)
modify doc for ParameterList and LayerList (#27609)

* modify doc for ParameterList and LayerList, test=develop

* add empty line after code-block, test=develop
luotao1/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,PaddlePaddle/Paddle,luotao1/Paddle,luotao1/Paddle