[Doc]Add MXNET_BACKWARD_DO_MIRROR in MXNet environment description (#4300)
[Doc]Add MXNET_BACKWARD_DO_MIRROR in MXNet environment description (#4300)

* [Doc]Add MXNET_BACKWARD_DO_MIRROR in MXNet environment description

* fix wrong description about mirror

pluskid/mxnet,tornadomeet/mxnet,apaleyes/mxnet,reminisce/mxnet,larroy/mxnet,xcgoner/dist-mxnet,eric-haibin-lin/mxnet,antoan2/incubator-mxnet,dmlc/mxnet,precedenceguo/mxnet,TuSimple/mxnet,gautamkmr/incubator-mxnet,antoan2/incubator-mxnet,navrasio/mxnet,fullfanta/mxnet,wangyum/mxnet,ForkedReposBak/mxnet,indhub/mxnet,pluskid/mxnet,eric-haibin-lin/mxnet,Ldpe2G/mxnet,Ldpe2G/mxnet,yuruofeifei/mxnet,EvanzzzZ/mxnet,Northrend/mxnet,Ldpe2G/mxnet,TuSimple/mxnet,mbaijal/incubator-mxnet,yuruofeifei/mxnet,indhub/mxnet,Guneet-Dhillon/mxnet,xcgoner/dist-mxnet,Ldpe2G/mxnet,Northrend/mxnet,jiajiechen/mxnet,likelyzhao/mxnet,piiswrong/mxnet,precedenceguo/mxnet,weleen/mxnet,madjam/mxnet,EvanzzzZ/mxnet,madjam/mxnet,jiajiechen/mxnet,lxn2/mxnet,apaleyes/mxnet,navrasio/mxnet,ptrendx/mxnet,luoyetx/mxnet,larroy/mxnet,pluskid/mxnet,szha/mxnet,apaleyes/mxnet,larroy/mxnet,Ldpe2G/mxnet,Prasad9/incubator-mxnet,crazy-cat/incubator-mxnet,szha/mxnet,saurabh3949/mxnet,formath/mxnet,ShownX/incubator-mxnet,apaleyes/mxnet,yuruofeifei/mxnet,kkk669/mxnet,piiswrong/mxnet,jamesliu/mxnet,vikingMei/mxnet,solin319/incubator-mxnet,wangyum/mxnet,DR08/mxnet,nicklhy/mxnet,hotpxl/mxnet,solin319/incubator-mxnet,stefanhenneking/mxnet,solin319/incubator-mxnet,precedenceguo/mxnet,Mega-DatA-Lab/mxnet,hpi-xnor/BMXNet,madjam/mxnet,mbaijal/incubator-mxnet,crazy-cat/incubator-mxnet,rishita/mxnet,ykim362/mxnet,jiajiechen/mxnet,EvanzzzZ/mxnet,antoan2/incubator-mxnet,yuruofeifei/mxnet,lxn2/mxnet,danithaca/mxnet,Ldpe2G/mxnet,antoan2/incubator-mxnet,wangyum/mxnet,crazy-cat/incubator-mxnet,Prasad9/incubator-mxnet,arikpoz/mxnet,luoyetx/mxnet,yajiedesign/mxnet,jamesliu/mxnet,fullfanta/mxnet,vikingMei/mxnet,hotpxl/mxnet,vikingMei/mxnet,yajiedesign/mxnet,saurabh3949/mxnet,crazy-cat/incubator-mxnet,pluskid/mxnet,ptrendx/mxnet,DR08/mxnet,coder-james/mxnet,dmlc/mxnet,vikingMei/mxnet,hotpxl/mxnet,luoyetx/mxnet,gautamkmr/incubator-mxnet,szha/mxnet,danithaca/mxnet,tlby/mxnet,sxjscience/mxnet,kevinthesun/mxnet,ykim362/mxnet,smolix/incubator-mxnet,arank/mxnet,hpi-xnor/BMXNet,solin319/incubator-mxnet,mbaijal/incubator-mxnet,ForkedReposBak/mxnet,leezu/mxnet,DickJC123/mxnet,leezu/mxnet,lxn2/mxnet,thirdwing/mxnet,wangyum/mxnet,arikpoz/mxnet,smolix/incubator-mxnet,jamesliu/mxnet,hesseltuinhof/mxnet,reminisce/mxnet,reminisce/mxnet,weleen/mxnet,thirdwing/mxnet,leezu/mxnet,ptrendx/mxnet,madjam/mxnet,zhreshold/mxnet,CodingCat/mxnet,yajiedesign/mxnet,likelyzhao/mxnet,ptrendx/mxnet,leezu/mxnet,Northrend/mxnet,Prasad9/incubator-mxnet,zhreshold/mxnet,kkk669/mxnet,mbaijal/incubator-mxnet,larroy/mxnet,hesseltuinhof/mxnet,formath/mxnet,kevinthesun/mxnet,arikpoz/mxnet,arikpoz/mxnet,LinkHS/incubator-mxnet,danithaca/mxnet,navrasio/mxnet,ptrendx/mxnet,hotpxl/mxnet,Northrend/mxnet,piiswrong/mxnet,LinkHS/incubator-mxnet,jermainewang/mxnet,gautamkmr/incubator-mxnet,dmlc/mxnet,TuSimple/mxnet,ptrendx/mxnet,crazy-cat/incubator-mxnet,sxjscience/mxnet,sergeykolychev/mxnet,CodingCat/mxnet,Ldpe2G/mxnet,tlby/mxnet,solin319/incubator-mxnet,DickJC123/mxnet,Prasad9/incubator-mxnet,danithaca/mxnet,Guneet-Dhillon/mxnet,formath/mxnet,apaleyes/mxnet,crazy-cat/incubator-mxnet,saurabh3949/mxnet,yajiedesign/mxnet,yuruofeifei/mxnet,tornadomeet/mxnet,coder-james/mxnet,fullfanta/mxnet,kkk669/mxnet,stefanhenneking/mxnet,lxn2/mxnet,gautamkmr/incubator-mxnet,pluskid/mxnet,pluskid/mxnet,sergeykolychev/mxnet,ZihengJiang/mxnet,kevinthesun/mxnet,Prasad9/incubator-mxnet,formath/mxnet,Ldpe2G/mxnet,TuSimple/mxnet,vikingMei/mxnet,Guneet-Dhillon/mxnet,Mega-DatA-Lab/mxnet,indhub/mxnet,smolix/incubator-mxnet,tlby/mxnet,reminisce/mxnet,apache/incubator-mxnet,apaleyes/mxnet,stefanhenneking/mxnet,Mega-DatA-Lab/mxnet,solin319/incubator-mxnet,leezu/mxnet,apaleyes/mxnet,DR08/mxnet,nicklhy/mxnet,xcgoner/dist-mxnet,sergeykolychev/mxnet,jermainewang/mxnet,tlby/mxnet,reminisce/mxnet,precedenceguo/mxnet,hpi-xnor/BMXNet,wangyum/mxnet,wangyum/mxnet,lxn2/mxnet,luoyetx/mxnet,smolix/incubator-mxnet,jiajiechen/mxnet,crazy-cat/incubator-mxnet,xcgoner/dist-mxnet,formath/mxnet,Mega-DatA-Lab/mxnet,indhub/mxnet,smolix/incubator-mxnet,luoyetx/mxnet,sergeykolychev/mxnet,arank/mxnet,CodingCat/mxnet,piiswrong/mxnet,rahul003/mxnet,tlby/mxnet,fullfanta/mxnet,gautamkmr/incubator-mxnet,mbaijal/incubator-mxnet,ykim362/mxnet,Guneet-Dhillon/mxnet,zhreshold/mxnet,larroy/mxnet,nicklhy/mxnet,TuSimple/mxnet,eric-haibin-lin/mxnet,reminisce/mxnet,zhreshold/mxnet,saurabh3949/mxnet,antoan2/incubator-mxnet,TuSimple/mxnet,tornadomeet/mxnet,hesseltuinhof/mxnet,stefanhenneking/mxnet,rahul003/mxnet,hotpxl/mxnet,indhub/mxnet,Northrend/mxnet,larroy/mxnet,indhub/mxnet,fullfanta/mxnet,szha/mxnet,ykim362/mxnet,tlby/mxnet,sxjscience/mxnet,mbaijal/incubator-mxnet,hpi-xnor/BMXNet,hesseltuinhof/mxnet,kkk669/mxnet,rishita/mxnet,luoyetx/mxnet,kevinthesun/mxnet,thirdwing/mxnet,EvanzzzZ/mxnet,jamesliu/mxnet,sxjscience/mxnet,LinkHS/incubator-mxnet,arank/mxnet,coder-james/mxnet,Guneet-Dhillon/mxnet,thirdwing/mxnet,likelyzhao/mxnet,sergeykolychev/mxnet,EvanzzzZ/mxnet,CodingCat/mxnet,hpi-xnor/BMXNet,yuruofeifei/mxnet,mbaijal/incubator-mxnet,xcgoner/dist-mxnet,rishita/mxnet,madjam/mxnet,hotpxl/mxnet,antoan2/incubator-mxnet,yuruofeifei/mxnet,ZihengJiang/mxnet,weleen/mxnet,CodingCat/mxnet,jamesliu/mxnet,precedenceguo/mxnet,navrasio/mxnet,ptrendx/mxnet,rahul003/mxnet,saurabh3949/mxnet,vikingMei/mxnet,jennyzhang0215/incubator-mxnet,yuruofeifei/mxnet,tlby/mxnet,Northrend/mxnet,hotpxl/mxnet,Prasad9/incubator-mxnet,danithaca/mxnet,ForkedReposBak/mxnet,Mega-DatA-Lab/mxnet,tornadomeet/mxnet,hesseltuinhof/mxnet,arikpoz/mxnet,yajiedesign/mxnet,luoyetx/mxnet,thirdwing/mxnet,indhub/mxnet,likelyzhao/mxnet,navrasio/mxnet,likelyzhao/mxnet,pluskid/mxnet,arikpoz/mxnet,rahul003/mxnet,luoyetx/mxnet,mbaijal/incubator-mxnet,jermainewang/mxnet,rahul003/mxnet,weleen/mxnet,LinkHS/incubator-mxnet,ShownX/incubator-mxnet,stefanhenneking/mxnet,arikpoz/mxnet,navrasio/mxnet,nicklhy/mxnet,piiswrong/mxnet,jennyzhang0215/incubator-mxnet,mbaijal/incubator-mxnet,lxn2/mxnet,saurabh3949/mxnet,sxjscience/mxnet,yajiedesign/mxnet,arikpoz/mxnet,danithaca/mxnet,nicklhy/mxnet,arank/mxnet,TuSimple/mxnet,likelyzhao/mxnet,ShownX/incubator-mxnet,DickJC123/mxnet,gautamkmr/incubator-mxnet,ZihengJiang/mxnet,jiajiechen/mxnet,DR08/mxnet,smolix/incubator-mxnet,indhub/mxnet,lxn2/mxnet,TuSimple/mxnet,rahul003/mxnet,antoan2/incubator-mxnet,jermainewang/mxnet,kevinthesun/mxnet,reminisce/mxnet,zhreshold/mxnet,DickJC123/mxnet,jiajiechen/mxnet,fullfanta/mxnet,stefanhenneking/mxnet,Ldpe2G/mxnet,madjam/mxnet,danithaca/mxnet,coder-james/mxnet,solin319/incubator-mxnet,reminisce/mxnet,dmlc/mxnet,larroy/mxnet,dmlc/mxnet,hpi-xnor/BMXNet,xcgoner/dist-mxnet,coder-james/mxnet,lxn2/mxnet,xcgoner/dist-mxnet,arikpoz/mxnet,ZihengJiang/mxnet,madjam/mxnet,jamesliu/mxnet,vikingMei/mxnet,yajiedesign/mxnet,kkk669/mxnet,navrasio/mxnet,sergeykolychev/mxnet,yuruofeifei/mxnet,gautamkmr/incubator-mxnet,LinkHS/incubator-mxnet,weleen/mxnet,ykim362/mxnet,apache/incubator-mxnet,jennyzhang0215/incubator-mxnet,szha/mxnet,leezu/mxnet,precedenceguo/mxnet,ZihengJiang/mxnet,ShownX/incubator-mxnet,navrasio/mxnet,DR08/mxnet,jennyzhang0215/incubator-mxnet,saurabh3949/mxnet,ForkedReposBak/mxnet,szha/mxnet,hesseltuinhof/mxnet,Northrend/mxnet,dmlc/mxnet,hpi-xnor/BMXNet,zhreshold/mxnet,hotpxl/mxnet,jamesliu/mxnet,eric-haibin-lin/mxnet,zhreshold/mxnet,ykim362/mxnet,leezu/mxnet,ShownX/incubator-mxnet,weleen/mxnet,Guneet-Dhillon/mxnet,tlby/mxnet,LinkHS/incubator-mxnet,CodingCat/mxnet,piiswrong/mxnet,Mega-DatA-Lab/mxnet,larroy/mxnet,weleen/mxnet,thirdwing/mxnet,ptrendx/mxnet,hpi-xnor/BMXNet,danithaca/mxnet,rahul003/mxnet,navrasio/mxnet,Mega-DatA-Lab/mxnet,kkk669/mxnet,arank/mxnet,reminisce/mxnet,ZihengJiang/mxnet,formath/mxnet,formath/mxnet,ptrendx/mxnet,ZihengJiang/mxnet,thirdwing/mxnet,smolix/incubator-mxnet,coder-james/mxnet,hotpxl/mxnet,szha/mxnet,jennyzhang0215/incubator-mxnet,leezu/mxnet,arank/mxnet,hesseltuinhof/mxnet,ShownX/incubator-mxnet,nicklhy/mxnet,DR08/mxnet,jennyzhang0215/incubator-mxnet,formath/mxnet,hpi-xnor/BMXNet,madjam/mxnet,LinkHS/incubator-mxnet,fullfanta/mxnet,kevinthesun/mxnet,yajiedesign/mxnet,kkk669/mxnet,precedenceguo/mxnet,tornadomeet/mxnet,kkk669/mxnet,CodingCat/mxnet,jennyzhang0215/incubator-mxnet,tornadomeet/mxnet,jennyzhang0215/incubator-mxnet,weleen/mxnet,Guneet-Dhillon/mxnet,solin319/incubator-mxnet,arank/mxnet,kkk669/mxnet,rishita/mxnet,arank/mxnet,ForkedReposBak/mxnet,TuSimple/mxnet,ykim362/mxnet,ZihengJiang/mxnet,weleen/mxnet,coder-james/mxnet,hesseltuinhof/mxnet,tlby/mxnet,eric-haibin-lin/mxnet,TuSimple/mxnet,jermainewang/mxnet,Mega-DatA-Lab/mxnet,apache/incubator-mxnet,rishita/mxnet,stefanhenneking/mxnet,ForkedReposBak/mxnet,CodingCat/mxnet,eric-haibin-lin/mxnet,dmlc/mxnet,eric-haibin-lin/mxnet,kevinthesun/mxnet,nicklhy/mxnet,solin319/incubator-mxnet,ShownX/incubator-mxnet,ZihengJiang/mxnet,precedenceguo/mxnet,LinkHS/incubator-mxnet,sxjscience/mxnet,coder-james/mxnet,formath/mxnet,Guneet-Dhillon/mxnet,jermainewang/mxnet,rahul003/mxnet,kevinthesun/mxnet,wangyum/mxnet,smolix/incubator-mxnet,gautamkmr/incubator-mxnet,zhreshold/mxnet,stefanhenneking/mxnet,ptrendx/mxnet,apaleyes/mxnet,rahul003/mxnet,sergeykolychev/mxnet,likelyzhao/mxnet,CodingCat/mxnet,nicklhy/mxnet,zhreshold/mxnet,EvanzzzZ/mxnet,DR08/mxnet,danithaca/mxnet,szha/mxnet,Northrend/mxnet,sxjscience/mxnet,DR08/mxnet,EvanzzzZ/mxnet,jermainewang/mxnet,lxn2/mxnet,tornadomeet/mxnet,Guneet-Dhillon/mxnet,eric-haibin-lin/mxnet,arank/mxnet,apache/incubator-mxnet,jamesliu/mxnet,dmlc/mxnet,piiswrong/mxnet,saurabh3949/mxnet,indhub/mxnet,jermainewang/mxnet,thirdwing/mxnet,piiswrong/mxnet,vikingMei/mxnet,EvanzzzZ/mxnet,crazy-cat/incubator-mxnet,jiajiechen/mxnet,jamesliu/mxnet,wangyum/mxnet,EvanzzzZ/mxnet,wangyum/mxnet,likelyzhao/mxnet,ShownX/incubator-mxnet,apaleyes/mxnet,smolix/incubator-mxnet,jennyzhang0215/incubator-mxnet,xcgoner/dist-mxnet,tornadomeet/mxnet,madjam/mxnet,likelyzhao/mxnet,jiajiechen/mxnet,thirdwing/mxnet,piiswrong/mxnet,jiajiechen/mxnet,sergeykolychev/mxnet,dmlc/mxnet,tlby/mxnet,xcgoner/dist-mxnet,hesseltuinhof/mxnet,rishita/mxnet,vikingMei/mxnet,rishita/mxnet,jermainewang/mxnet,ykim362/mxnet,szha/mxnet,ShownX/incubator-mxnet,coder-james/mxnet,rishita/mxnet,sergeykolychev/mxnet,ykim362/mxnet,larroy/mxnet,Prasad9/incubator-mxnet,crazy-cat/incubator-mxnet,ForkedReposBak/mxnet,fullfanta/mxnet,tornadomeet/mxnet,Prasad9/incubator-mxnet,kevinthesun/mxnet,LinkHS/incubator-mxnet,saurabh3949/mxnet,rishita/mxnet,Northrend/mxnet,Mega-DatA-Lab/mxnet,nicklhy/mxnet,sxjscience/mxnet,reminisce/mxnet,gautamkmr/incubator-mxnet,pluskid/mxnet,antoan2/incubator-mxnet,Prasad9/incubator-mxnet,fullfanta/mxnet,luoyetx/mxnet,leezu/mxnet,precedenceguo/mxnet,antoan2/incubator-mxnet,DR08/mxnet,sxjscience/mxnet,ForkedReposBak/mxnet,ForkedReposBak/mxnet,yajiedesign/mxnet,stefanhenneking/mxnet,eric-haibin-lin/mxnet,pluskid/mxnet