Refine doc for smooth l1 loss op.
Refine doc for smooth l1 loss op.

reyoung/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,jacquesqiao/Paddle,Canpio/Paddle,putcn/Paddle,baidu/Paddle,jacquesqiao/Paddle,QiJune/Paddle,baidu/Paddle,chengduoZH/Paddle,luotao1/Paddle,lcy-seso/Paddle,lcy-seso/Paddle,pkuyym/Paddle,PaddlePaddle/Paddle,lcy-seso/Paddle,jacquesqiao/Paddle,chengduoZH/Paddle,pkuyym/Paddle,reyoung/Paddle,QiJune/Paddle,tensor-tang/Paddle,jacquesqiao/Paddle,Canpio/Paddle,reyoung/Paddle,reyoung/Paddle,luotao1/Paddle,Canpio/Paddle,PaddlePaddle/Paddle,reyoung/Paddle,luotao1/Paddle,PaddlePaddle/Paddle,putcn/Paddle,Canpio/Paddle,tensor-tang/Paddle,jacquesqiao/Paddle,Canpio/Paddle,PaddlePaddle/Paddle,Canpio/Paddle,PaddlePaddle/Paddle,putcn/Paddle,baidu/Paddle,pkuyym/Paddle,jacquesqiao/Paddle,luotao1/Paddle,putcn/Paddle,pkuyym/Paddle,baidu/Paddle,Canpio/Paddle,reyoung/Paddle,pkuyym/Paddle,luotao1/Paddle,chengduoZH/Paddle,chengduoZH/Paddle,Canpio/Paddle,PaddlePaddle/Paddle,QiJune/Paddle,QiJune/Paddle,QiJune/Paddle,pkuyym/Paddle,putcn/Paddle,putcn/Paddle,tensor-tang/Paddle,luotao1/Paddle,tensor-tang/Paddle,baidu/Paddle,tensor-tang/Paddle,chengduoZH/Paddle,lcy-seso/Paddle,lcy-seso/Paddle,QiJune/Paddle,lcy-seso/Paddle