net: document GODEBUG=netdns=xxx settings
net: document GODEBUG=netdns=xxx settings

Fixes #12191.

Change-Id: I5c7659ccb0566dad3613041d9e76be87ceacae61
Reviewed-on: https://go-review.googlesource.com/13700
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

binarycrusader/go,dterei/go,ganboing/go-esx,albertjin/golang-go,cloudflare/go,arschles/golang-go,zcwfeng/go,4ad/go,goodlang/good,dafyddcrosby/go,cainiaocome/go,JioCloud/go,Jordanzuo/go,albertjin/golang-go,jdhenke/go,snowsnail/go,ifagnosticism/go,myPublicGit/go,ifagnosticism/go,danycoro/go,mikekap/go,Samurais/go,theass/go,likesea/go,devpadawan/go,roth1002/go,ichu501/go,jacobhaven/go,nawawi/go,yangzhongj/go,alex-zhang/go,cyberroadie/go,Endika/go,codestation/go,ichu501/go,snowsnail/go,cyberroadie/go,zcwfeng/go,stormltf/go,Jordanzuo/go,Samurais/go,Cofyc/go,Cofyc/go,rachtsingh/gogojuice,vsdutka/go,slavau/go,greyhwndz/go,cookerzhu/go,jacobhaven/go,odeke-em/go,jonahglover/go,binarycrusader/go,rachtsingh/gogojuice,rentongzhang/go,Cofyc/go,dpatel06/go,cloudflare/go,deft-code/go,ImJasonH/go,beealone/go,donnel6809/go,jdeng/go,CAFxX/go,lihuanghai/go,jasonxiong/go,dafyddcrosby/go,theif519/go,vsekhar/elastic-go,kim-racktop/go,kakuhiroshi/go,ImJasonH/go,polyverse-security/go,xujianhai/go,kim-racktop/go,monetate/go,ganboing/go-esx,malvira/go,honsiorovskyi/go,qskycolor/go,theass/go,kim-racktop/go,yangzhongj/go,myPublicGit/go,mikekap/go,vsdutka/go,dterei/go,AALEKH/go,theass/go,joshsammut/go,ifagnosticism/go,jonahglover/go,greyhwndz/go,slavau/go,qskycolor/go,myPublicGit/go,wangfakang/go-1,4ad/go,suncycheng/go,CarterTsai/go,danycoro/go,jefby/go,mwhudson/go,dpatel06/go,willjunspecial/go,sndnvaps/go,uileyar/go,likesea/go,theass/go,CAFxX/go,honsiorovskyi/go,cookerzhu/go,kakuhiroshi/go,nawawi/go,4ad/go,sinuos/go,vsdutka/go,CarterTsai/go,XuYunnan/go,vishsingh/go,jonahglover/go,fjballest/golang,Pulgafree/go,joshsammut/go,alex-zhang/go,Charlesdong/go-1,jefby/go,jonahglover/go,jonasi/go,nakedgc/go,mdp/go,fjballest/golang,lihuanghai/go,xiaobodu/go,Endika/go,NunoEdgarGub1/go,chinanjjohn2012/go,joshsammut/go,nawawi/go,momchil-velikov/go,golang/go,deft-code/go,kim-racktop/go,xiuzhifu/go,danielsoro/go,dafyddcrosby/go,snowsnail/go,greyhwndz/go,xiaobodu/go,WIZARD-CXY/go,greyhwndz/go,Cofyc/go,ganboing/go-esx,mwhudson/go,devpadawan/go,Jordanzuo/go,xiuzhifu/go,leeric92/go,vishsingh/go,momchil-velikov/go,famorted/go,ganboing/go-esx,goodlang/good,dpatel06/go,famorted/go,jonathanmarvens/go,suncycheng/go,vanloswang/go,arschles/go-1,Young55555/go,cainiaocome/go,CarterTsai/go,vishsingh/go,gdey/go,Pulgafree/go,WIZARD-CXY/go,skatsuta/go,odeke-em/go,zhangg/go,rentongzhang/go,cainiaocome/go,constantine001/go,monetate/go,CiscoCloud/go,jacobxk/go,fjballest/golang,malvira/go,linux-on-ibm-z/go,XuYunnan/go,shines77/go,josharian/go,Stan-Lin-/go,golang/go,jdhenke/go,greyhwndz/go,xujianhai/go,greyhwndz/go,chinanjjohn2012/go,arschles/golang-go,fjballest/golang,rachtsingh/gogojuice,ganboing/go-esx,theif519/go,Jordanzuo/go,jonasi/go,cookerzhu/go,josharian/go,suncycheng/go,alash3al/go,myPublicGit/go,jefby/go,NichoZhang/go,arschles/go-1,josharian/go,linux-on-ibm-z/go,jacobhaven/go,alash3al/go,kakuhiroshi/go,rachtsingh/gogojuice,mdp/go,cainiaocome/go,theass/go,odeke-em/go,donnel6809/go,malvira/go,arschles/go-1,GeorgiCodes/go,goodlang/good,donnel6809/go,GeorgiCodes/go,monetate/go,jacobhaven/go,yasarkunduz/go,odeke-em/go,alash3al/go,XuYunnan/go,NichoZhang/go,mgyenik/go,jacobxk/go,CiscoCloud/go,uileyar/go,ImJasonH/go,mgyenik/go,linux-on-ibm-z/go,NunoEdgarGub1/go,ifagnosticism/go,dpatel06/go,polyverse-security/go,polyverse-security/go,nawawi/go,xiuzhifu/go,slavau/go,snowsnail/go,cainiaocome/go,NichoZhang/go,jonathanmarvens/go,jonathanmarvens/go,roth1002/go,honsiorovskyi/go,leeric92/go,cookerzhu/go,sndnvaps/go,linux-on-ibm-z/go,samanalysis/go,jonasi/go,sndnvaps/go,sinuos/go,skatsuta/go,deft-code/go,akutz/go,Stan-Lin-/go,alex-zhang/go,dafyddcrosby/go,Badredapple/go,mdp/go,binarycrusader/go,famorted/go,derekmarcotte/go,mbrukman/go,undergrowthlinear/go,mikekap/go,constantine001/go,albertjin/golang-go,ImJasonH/go,NunoEdgarGub1/go,jacobhaven/go,ObjectiveJoe/go,fjballest/golang,jdhenke/go,mikekap/go,GeorgiCodes/go,likesea/go,rachtsingh/gogojuice,jacobxk/go,zcwfeng/go,alash3al/go,mbrukman/go,CarterTsai/go,Pulgafree/go,arschles/go-1,malvira/go,Samurais/go,rachtsingh/gogojuice,odeke-em/go,slavau/go,ichu501/go,arschles/golang-go,nakedgc/go,NunoEdgarGub1/go,nakedgc/go,vsdutka/go,Charlesdong/go-1,derekmarcotte/go,zhangg/go,albertjin/golang-go,jdhenke/go,Endika/go,theass/go,GeorgiCodes/go,CiscoCloud/go,stormltf/go,NichoZhang/go,constantine001/go,ericsnowcurrently/go,CarterTsai/go,willjunspecial/go,zcwfeng/go,linux-on-ibm-z/go,zouchao2010/go,roth1002/go,dafyddcrosby/go,zouchao2010/go,ichu501/go,Pulgafree/go,momchil-velikov/go,shines77/go,Endika/go,danielsoro/go,vsekhar/elastic-go,cloudflare/go,jdeng/go,jdeng/go,yangzhongj/go,suncycheng/go,kim-racktop/go,leeric92/go,ichu501/go,Charlesdong/go-1,slavau/go,Jordanzuo/go,jonahglover/go,CiscoCloud/go,danielsoro/go,monetate/go,WIZARD-CXY/go,zouchao2010/go,mgyenik/go,b54898533/go,uileyar/go,sinuos/go,4ad/go,JioCloud/go,danielsoro/go,albertjin/golang-go,sndnvaps/go,jasonxiong/go,derekmarcotte/go,jasonxiong/go,xiaobodu/go,CAFxX/go,ObjectiveJoe/go,ichu501/go,alex-zhang/go,cloudflare/go,sinuos/go,likesea/go,danycoro/go,jonasi/go,WIZARD-CXY/go,dterei/go,b54898533/go,dafyddcrosby/go,mikekap/go,deft-code/go,Endika/go,qskycolor/go,polyverse-security/go,CAFxX/go,devpadawan/go,deft-code/go,beealone/go,zcwfeng/go,Samurais/go,jdeng/go,roth1002/go,yasarkunduz/go,josharian/go,shines77/go,jacobxk/go,joshsammut/go,Samurais/go,AALEKH/go,qskycolor/go,undergrowthlinear/go,akutz/go,cyberroadie/go,stormltf/go,constantine001/go,cyberroadie/go,leeric92/go,jacobsa/go,mbrukman/go,alash3al/go,zcwfeng/go,codestation/go,Samurais/go,CarterTsai/go,joshsammut/go,beealone/go,derekmarcotte/go,golang/go,linux-on-ibm-z/go,jacobsa/go,Young55555/go,josharian/go,jacobxk/go,qskycolor/go,CiscoCloud/go,danycoro/go,CAFxX/go,suncycheng/go,xiuzhifu/go,Stan-Lin-/go,jonasi/go,nakedgc/go,goodlang/good,4ad/go,Pulgafree/go,alash3al/go,nakedgc/go,zouchao2010/go,CarterTsai/go,samanalysis/go,devpadawan/go,vishsingh/go,jacobsa/go,JioCloud/go,AALEKH/go,lihuanghai/go,malvira/go,Pulgafree/go,vsekhar/elastic-go,kakuhiroshi/go,vsdutka/go,akutz/go,mdp/go,dterei/go,linux-on-ibm-z/go,CiscoCloud/go,ifagnosticism/go,jdeng/go,mdp/go,b54898533/go,vsdutka/go,Cofyc/go,cyberroadie/go,ericsnowcurrently/go,xujianhai/go,rentongzhang/go,mwhudson/go,jacobxk/go,xiaobodu/go,xiaobodu/go,willjunspecial/go,yasarkunduz/go,dterei/go,roth1002/go,stormltf/go,lihuanghai/go,polyverse-security/go,yangzhongj/go,honsiorovskyi/go,nawawi/go,akutz/go,leeric92/go,goodlang/good,golang/go,gdey/go,akutz/go,uileyar/go,vanloswang/go,derekmarcotte/go,beealone/go,xujianhai/go,lihuanghai/go,slavau/go,jacobhaven/go,vsdutka/go,ObjectiveJoe/go,nawawi/go,ObjectiveJoe/go,albertjin/golang-go,arschles/golang-go,myPublicGit/go,goodlang/good,codestation/go,famorted/go,ericsnowcurrently/go,jacobsa/go,JioCloud/go,honsiorovskyi/go,beealone/go,malvira/go,4ad/go,jonasi/go,CiscoCloud/go,vanloswang/go,lihuanghai/go,Stan-Lin-/go,XuYunnan/go,cyberroadie/go,jefby/go,Cofyc/go,theif519/go,ichu501/go,willjunspecial/go,cainiaocome/go,momchil-velikov/go,wangfakang/go-1,rachtsingh/gogojuice,dafyddcrosby/go,nakedgc/go,Charlesdong/go-1,Pulgafree/go,rentongzhang/go,famorted/go,undergrowthlinear/go,devpadawan/go,monetate/go,theass/go,vanloswang/go,vishsingh/go,likesea/go,CAFxX/go,jdhenke/go,constantine001/go,XuYunnan/go,NichoZhang/go,donnel6809/go,ObjectiveJoe/go,zhangg/go,mbrukman/go,stormltf/go,qskycolor/go,Stan-Lin-/go,dpatel06/go,jacobsa/go,yangzhongj/go,jonathanmarvens/go,cloudflare/go,akutz/go,honsiorovskyi/go,XuYunnan/go,yangzhongj/go,theass/go,jonathanmarvens/go,Badredapple/go,uileyar/go,skatsuta/go,josharian/go,AALEKH/go,b54898533/go,jonathanmarvens/go,mgyenik/go,myPublicGit/go,jdhenke/go,josharian/go,Jordanzuo/go,goodlang/good,polyverse-security/go,cookerzhu/go,albertjin/golang-go,chinanjjohn2012/go,nakedgc/go,leeric92/go,ganboing/go-esx,theif519/go,Charlesdong/go-1,stormltf/go,odeke-em/go,CarterTsai/go,devpadawan/go,AALEKH/go,myPublicGit/go,akutz/go,xiuzhifu/go,arschles/golang-go,Pulgafree/go,wangfakang/go-1,momchil-velikov/go,jonathanmarvens/go,shines77/go,joshsammut/go,Samurais/go,skatsuta/go,xiuzhifu/go,golang/go,miolini/go,mgyenik/go,monetate/go,miolini/go,jdhenke/go,mbrukman/go,joshsammut/go,fjballest/golang,momchil-velikov/go,sndnvaps/go,zouchao2010/go,jacobsa/go,dpatel06/go,skatsuta/go,theif519/go,GeorgiCodes/go,jasonxiong/go,yangzhongj/go,kim-racktop/go,jacobxk/go,ImJasonH/go,CAFxX/go,miolini/go,yasarkunduz/go,mwhudson/go,polyverse-security/go,Stan-Lin-/go,miolini/go,snowsnail/go,constantine001/go,jonathanmarvens/go,suncycheng/go,gdey/go,kakuhiroshi/go,Jordanzuo/go,WIZARD-CXY/go,derekmarcotte/go,xiaobodu/go,xujianhai/go,likesea/go,vanloswang/go,skatsuta/go,miolini/go,codestation/go,codestation/go,golang/go,nakedgc/go,binarycrusader/go,kim-racktop/go,malvira/go,vsekhar/elastic-go,mbrukman/go,ImJasonH/go,danycoro/go,dterei/go,samanalysis/go,skatsuta/go,sinuos/go,danielsoro/go,Endika/go,XuYunnan/go,jonahglover/go,momchil-velikov/go,shines77/go,jacobsa/go,famorted/go,gdey/go,Badredapple/go,undergrowthlinear/go,arschles/golang-go,chinanjjohn2012/go,suncycheng/go,vanloswang/go,xiaobodu/go,Endika/go,ericsnowcurrently/go,yasarkunduz/go,vsdutka/go,stormltf/go,lihuanghai/go,ericsnowcurrently/go,GeorgiCodes/go,Young55555/go,AALEKH/go,NichoZhang/go,sndnvaps/go,jefby/go,christopher-henderson/Go,NichoZhang/go,jacobxk/go,CAFxX/go,CiscoCloud/go,cookerzhu/go,honsiorovskyi/go,kakuhiroshi/go,alex-zhang/go,zhangg/go,likesea/go,rentongzhang/go,rentongzhang/go,binarycrusader/go,WIZARD-CXY/go,nawawi/go,ifagnosticism/go,miolini/go,xiaobodu/go,Cofyc/go,gdey/go,4ad/go,Charlesdong/go-1,codestation/go,mbrukman/go,albertjin/golang-go,Young55555/go,sinuos/go,jefby/go,jasonxiong/go,constantine001/go,danycoro/go,b54898533/go,undergrowthlinear/go,mikekap/go,undergrowthlinear/go,ganboing/go-esx,mgyenik/go,gdey/go,gdey/go,undergrowthlinear/go,cyberroadie/go,samanalysis/go,jonasi/go,jasonxiong/go,ObjectiveJoe/go,WIZARD-CXY/go,famorted/go,donnel6809/go,mdp/go,cookerzhu/go,ObjectiveJoe/go,cloudflare/go,qskycolor/go,donnel6809/go,ericsnowcurrently/go,Charlesdong/go-1,gdey/go,jefby/go,ImJasonH/go,NunoEdgarGub1/go,alash3al/go,Cofyc/go,cookerzhu/go,shines77/go,JioCloud/go,vsekhar/elastic-go,mdp/go,slavau/go,joshsammut/go,ichu501/go,vishsingh/go,samanalysis/go,mikekap/go,ifagnosticism/go,arschles/golang-go,roth1002/go,jonasi/go,stormltf/go,willjunspecial/go,zhangg/go,zouchao2010/go,dafyddcrosby/go,golang/go,arschles/go-1,mikekap/go,cainiaocome/go,mgyenik/go,jdeng/go,jdeng/go,beealone/go,mdp/go,danielsoro/go,jefby/go,uileyar/go,goodlang/good,donnel6809/go,alex-zhang/go,alex-zhang/go,mwhudson/go,wangfakang/go-1,mwhudson/go,NichoZhang/go,zouchao2010/go,lihuanghai/go,Jordanzuo/go,snowsnail/go,binarycrusader/go,odeke-em/go,wangfakang/go-1,rachtsingh/gogojuice,deft-code/go,jonahglover/go,snowsnail/go,cyberroadie/go,Stan-Lin-/go,binarycrusader/go,danycoro/go,zhangg/go,danielsoro/go,vishsingh/go,cloudflare/go,NunoEdgarGub1/go,suncycheng/go,miolini/go,zhangg/go,b54898533/go,linux-on-ibm-z/go,rentongzhang/go,mbrukman/go,shines77/go,uileyar/go,kim-racktop/go,zhangg/go,famorted/go,WIZARD-CXY/go,chinanjjohn2012/go,GeorgiCodes/go,xiuzhifu/go,malvira/go,xiuzhifu/go,Badredapple/go,samanalysis/go,ObjectiveJoe/go,jasonxiong/go,alex-zhang/go,mwhudson/go,myPublicGit/go,zouchao2010/go,kakuhiroshi/go,fjballest/golang,JioCloud/go,deft-code/go,sinuos/go,chinanjjohn2012/go,dpatel06/go,vsekhar/elastic-go,Badredapple/go,zcwfeng/go,codestation/go,b54898533/go,momchil-velikov/go,josharian/go,kakuhiroshi/go,nawawi/go,constantine001/go,samanalysis/go,leeric92/go,AALEKH/go,yangzhongj/go,JioCloud/go,mwhudson/go,undergrowthlinear/go,theif519/go,vsekhar/elastic-go,GeorgiCodes/go,xujianhai/go,slavau/go,binarycrusader/go,sndnvaps/go,b54898533/go,arschles/go-1,dterei/go,NunoEdgarGub1/go,cloudflare/go,derekmarcotte/go,xujianhai/go,JioCloud/go,alash3al/go,deft-code/go,jacobsa/go,NunoEdgarGub1/go,donnel6809/go,Endika/go,miolini/go,Stan-Lin-/go,ifagnosticism/go,monetate/go,arschles/go-1,xujianhai/go,sndnvaps/go,Samurais/go,Charlesdong/go-1,wangfakang/go-1,dterei/go,greyhwndz/go,Badredapple/go,devpadawan/go,ganboing/go-esx,vanloswang/go,samanalysis/go,AALEKH/go,rentongzhang/go,fjballest/golang,wangfakang/go-1,vsekhar/elastic-go,Badredapple/go,danycoro/go,leeric92/go,devpadawan/go,honsiorovskyi/go,jacobhaven/go,jdeng/go,vanloswang/go,Young55555/go,willjunspecial/go,chinanjjohn2012/go,cainiaocome/go,codestation/go,greyhwndz/go,polyverse-security/go,beealone/go,jasonxiong/go,derekmarcotte/go,dpatel06/go,roth1002/go,Young55555/go,roth1002/go,ericsnowcurrently/go,XuYunnan/go,beealone/go,odeke-em/go,snowsnail/go,danielsoro/go,zcwfeng/go,akutz/go,yasarkunduz/go,mgyenik/go,jdhenke/go,monetate/go,ImJasonH/go,Young55555/go,ericsnowcurrently/go,arschles/golang-go,willjunspecial/go,theif519/go,yasarkunduz/go,jacobhaven/go,chinanjjohn2012/go,4ad/go,theif519/go,willjunspecial/go,arschles/go-1,shines77/go,qskycolor/go,jonahglover/go,wangfakang/go-1,sinuos/go,vishsingh/go,Badredapple/go,skatsuta/go,yasarkunduz/go,likesea/go,golang/go,Young55555/go,uileyar/go