cmd/cgo: document how CGO_ENABLED works today
cmd/cgo: document how CGO_ENABLED works today

Fixes #9530.

Change-Id: Iadfc027c7164e3ba35adb5c67deb42b51d3498ca
Reviewed-on: https://go-review.googlesource.com/12603
Reviewed-by: Matthew Dempsky <e56ac2c500064424afac424d4b5e53bd0570a731@google.com>
Reviewed-by: Dave Cheney <bfcdf3e6ca6cef45543bfbb57509c92aec9a39fb@cheney.net>

ImJasonH/go,zouchao2010/go,mbrukman/go,jmptrader/go,skatsuta/go,xiuzhifu/go,xujianhai/go,NunoEdgarGub1/go,golang/go,mikekap/go,cloudflare/go,qskycolor/go,ichu501/go,yangzhongj/go,yasarkunduz/go,skotti/go,goodlang/good,NichoZhang/go,donnel6809/go,Stan-Lin-/go,kakuhiroshi/go,jacobxk/go,suncycheng/go,Jordanzuo/go,arschles/golang-go,dpatel06/go,shazow/go,qskycolor/go,linux-on-ibm-z/go,arschles/golang-go,jdhenke/go,Samurais/go,NunoEdgarGub1/go,samanalysis/go,Samurais/go,devpadawan/go,skotti/go,roth1002/go,rentongzhang/go,joshsammut/go,gdey/go,cainiaocome/go,roth1002/go,rentongzhang/go,joshsammut/go,parkdy/go,famorted/go,kevinc0825/go,jefby/go,ericsnowcurrently/go,b54898533/go,leeric92/go,xinhuang327/go,Jordanzuo/go,jonahglover/go,youprofit/go,sosop/go,arschles/go-1,donnel6809/go,xiuzhifu/go,CiscoCloud/go,fjballest/golang,CarterTsai/go,youprofit/go,Cofyc/go,jasonxiong/go,ErikDubbelboer/go,undergrowthlinear/go,kakuhiroshi/go,CodyGuo/go,deft-code/go,danycoro/go,samanalysis/go,WIZARD-CXY/go,Stan-Lin-/go,youprofit/go,zhangg/go,mgyenik/go,rentongzhang/go,ichu501/go,chinanjjohn2012/go,josharian/go,polyverse-security/go,AALEKH/go,slavau/go,honsiorovskyi/go,GeorgiCodes/go,vishsingh/go,derekmarcotte/go,mwhudson/go,CodyGuo/go,ErikDubbelboer/go,parkdy/go,jdeng/go,nakedgc/go,momchil-velikov/go,XuYunnan/go,g522342435/go,xujianhai/go,malvira/go,albertjin/golang-go,odeke-em/go,vishsingh/go,snowsnail/go,Cofyc/go,jonathanmarvens/go,shines77/go,andrewchambers/go,mwhudson/go,ErikDubbelboer/go,parkdy/go,skotti/go,mbrukman/go,myPublicGit/go,mbrukman/go,odeke-em/go,yangzhongj/go,suncycheng/go,constantine001/go,jameskumar/go,odeke-em/go,Charlesdong/go-1,dterei/go,zouchao2010/go,Charlesdong/go-1,mgyenik/go,alex-zhang/go,b54898533/go,nakedgc/go,malvira/go,slavau/go,akutz/go,malvira/go,beealone/go,chinanjjohn2012/go,CarterTsai/go,GeorgiCodes/go,famorted/go,zcwfeng/go,shazow/go,CiscoCloud/go,devpadawan/go,ganboing/go-esx,beealone/go,ifagnosticism/go,cloudrain21/go,qskycolor/go,Charlesdong/go-1,honsiorovskyi/go,likesea/go,uileyar/go,leeric92/go,uileyar/go,wangfakang/go-1,sndnvaps/go,zhangg/go,linux-on-ibm-z/go,sndnvaps/go,slavau/go,gohin/go,ganboing/go-esx,andrewchambers/go,rachtsingh/gogojuice,danycoro/go,Jordanzuo/go,akutz/go,constantine001/go,mgyenik/go,WIZARD-CXY/go,NichoZhang/go,akutz/go,jonahglover/go,binarycrusader/go,andrewchambers/go,snowsnail/go,codestation/go,cainiaocome/go,honsiorovskyi/go,CiscoCloud/go,zouchao2010/go,rachtsingh/gogojuice,sinuos/go,leeric92/go,willjunspecial/go,andrewchambers/go,xujianhai/go,monetate/go,uileyar/go,willjunspecial/go,arschles/go-1,baiyunping333/go,snowsnail/go,jacobsa/go,shazow/go,rachtsingh/gogojuice,cookerzhu/go,arschles/go-1,sinuos/go,JioCloud/go,suncycheng/go,jonasi/go,linux-on-ibm-z/go,ifagnosticism/go,jdhenke/go,Cofyc/go,jasonxiong/go,vanloswang/go,jefby/go,cloudflare/go,albertjin/golang-go,Charlesdong/go-1,cainiaocome/go,alex-zhang/go,suncycheng/go,likesea/go,snowsnail/go,chinanjjohn2012/go,goodlang/good,WIZARD-CXY/go,cloudrain21/go,ericsnowcurrently/go,gohin/go,Badredapple/go,mgyenik/go,polyverse-security/go,AALEKH/go,yuhengye/go,gohin/go,greyhwndz/go,dafyddcrosby/go,golang/go,danielsoro/go,xinhuang327/go,mgyenik/go,XuYunnan/go,CiscoCloud/go,polyverse-security/go,Samurais/go,jacobxk/go,jmptrader/go,Pulgafree/go,linux-on-ibm-z/go,beealone/go,Pulgafree/go,golang/go,samanalysis/go,linux-on-ibm-z/go,ObjectiveJoe/go,constantine001/go,jonahglover/go,shines77/go,arschles/go-1,goodlang/good,yangzhongj/go,derekmarcotte/go,ichu501/go,dafyddcrosby/go,springning/go,chinanjjohn2012/go,vsekhar/elastic-go,jameskumar/go,shazow/go,derekmarcotte/go,Cofyc/go,4ad/go,monetate/go,constantine001/go,vsdutka/go,qskycolor/go,Young55555/go,WIZARD-CXY/go,CodyGuo/go,josharian/go,roth1002/go,Stan-Lin-/go,vishsingh/go,dterei/go,myPublicGit/go,g522342435/go,suncycheng/go,willjunspecial/go,NichoZhang/go,yasarkunduz/go,momchil-velikov/go,jonathanmarvens/go,josharian/go,ObjectiveJoe/go,CarterTsai/go,momchil-velikov/go,cloudrain21/go,jonasi/go,cyberroadie/go,CAFxX/go,arschles/go-1,jdhenke/go,derekmarcotte/go,alash3al/go,golang/go,zouchao2010/go,jdhenke/go,jacobsa/go,nawawi/go,uileyar/go,mgyenik/go,cookerzhu/go,4ad/go,jameskumar/go,Badredapple/go,g522342435/go,CarterTsai/go,jacobsa/go,ImJasonH/go,sosop/go,theass/go,shines77/go,devpadawan/go,goodlang/good,GeorgiCodes/go,danielsoro/go,Cofyc/go,arschles/golang-go,jacobxk/go,Young55555/go,joshsammut/go,kim-racktop/go,deft-code/go,dterei/go,fjballest/golang,miolini/go,myPublicGit/go,shines77/go,cainiaocome/go,mwhudson/go,lihuanghai/go,jonathanmarvens/go,ichu501/go,xujianhai/go,nakedgc/go,yasarkunduz/go,vsekhar/elastic-go,Samurais/go,undergrowthlinear/go,jefby/go,jacobhaven/go,vishsingh/go,CodyGuo/go,mdp/go,constantine001/go,vsdutka/go,sosop/go,baiyunping333/go,ericsnowcurrently/go,nawawi/go,CiscoCloud/go,jmptrader/go,zouchao2010/go,slavau/go,yangzhongj/go,ericsnowcurrently/go,ganboing/go-esx,akutz/go,rentongzhang/go,jonathanmarvens/go,leeric92/go,josharian/go,polyverse-security/go,jmptrader/go,undergrowthlinear/go,CodyGuo/go,donnel6809/go,vsekhar/elastic-go,goodlang/good,goodlang/good,gdey/go,greyhwndz/go,zhangg/go,dafyddcrosby/go,AALEKH/go,NunoEdgarGub1/go,jacobsa/go,ImJasonH/go,ObjectiveJoe/go,Pulgafree/go,zcwfeng/go,lihuanghai/go,jacobhaven/go,binarycrusader/go,jacobsa/go,vanloswang/go,ichu501/go,youprofit/go,b54898533/go,shazow/go,cloudflare/go,xiaobodu/go,jonahglover/go,jdeng/go,devpadawan/go,derekmarcotte/go,stormltf/go,CarterTsai/go,nawawi/go,famorted/go,danycoro/go,binarycrusader/go,NunoEdgarGub1/go,dterei/go,chinanjjohn2012/go,baiyunping333/go,rentongzhang/go,mikekap/go,zcwfeng/go,jdeng/go,arschles/go-1,Badredapple/go,ErikDubbelboer/go,youprofit/go,jonahglover/go,theif519/go,xiaobodu/go,jasonxiong/go,jefby/go,golang/go,devpadawan/go,xiaobodu/go,constantine001/go,AALEKH/go,cyberroadie/go,b54898533/go,fjballest/golang,CAFxX/go,lihuanghai/go,Stan-Lin-/go,JioCloud/go,cloudflare/go,lihuanghai/go,codestation/go,xiaobodu/go,jmptrader/go,sosop/go,CAFxX/go,golang/go,mdp/go,fjballest/golang,XuYunnan/go,nawawi/go,theass/go,yuhengye/go,yangzhongj/go,stormltf/go,jefby/go,jonasi/go,polyverse-security/go,sndnvaps/go,deft-code/go,joshsammut/go,jonathanmarvens/go,momchil-velikov/go,springning/go,GeorgiCodes/go,kevinc0825/go,4ad/go,dpatel06/go,rachtsingh/gogojuice,polyverse-security/go,roth1002/go,jdhenke/go,joshsammut/go,CAFxX/go,momchil-velikov/go,cyberroadie/go,jacobsa/go,parkdy/go,willjunspecial/go,fjballest/golang,miolini/go,cainiaocome/go,gdey/go,devpadawan/go,dpatel06/go,Samurais/go,malvira/go,Jordanzuo/go,ganboing/go-esx,derekmarcotte/go,Charlesdong/go-1,codestation/go,ErikDubbelboer/go,momchil-velikov/go,vsdutka/go,stormltf/go,dpatel06/go,mdp/go,mwhudson/go,odeke-em/go,mbrukman/go,malvira/go,kevinc0825/go,CarterTsai/go,jacobhaven/go,Charlesdong/go-1,uileyar/go,nakedgc/go,kim-racktop/go,golang/go,skotti/go,gdey/go,undergrowthlinear/go,sinuos/go,chinanjjohn2012/go,mbrukman/go,yuhengye/go,chinanjjohn2012/go,polyverse-security/go,monetate/go,roth1002/go,gdey/go,kim-racktop/go,malvira/go,kakuhiroshi/go,wangfakang/go-1,mwhudson/go,Badredapple/go,mikekap/go,jacobhaven/go,cloudflare/go,myPublicGit/go,uileyar/go,albertjin/golang-go,monetate/go,wangfakang/go-1,samanalysis/go,sndnvaps/go,derekmarcotte/go,vanloswang/go,slavau/go,danielsoro/go,vishsingh/go,alash3al/go,parkdy/go,jameskumar/go,jdeng/go,dafyddcrosby/go,ObjectiveJoe/go,CiscoCloud/go,springning/go,cainiaocome/go,yasarkunduz/go,Endika/go,nakedgc/go,skotti/go,cloudrain21/go,g522342435/go,g522342435/go,xiuzhifu/go,linux-on-ibm-z/go,jonathanmarvens/go,Samurais/go,kevinc0825/go,kim-racktop/go,binarycrusader/go,momchil-velikov/go,alex-zhang/go,theif519/go,nakedgc/go,samanalysis/go,momchil-velikov/go,xinhuang327/go,parkdy/go,jonahglover/go,goodlang/good,yuhengye/go,Pulgafree/go,Cofyc/go,xiaobodu/go,ichu501/go,snowsnail/go,skatsuta/go,mbrukman/go,leeric92/go,slavau/go,cookerzhu/go,sndnvaps/go,constantine001/go,danycoro/go,youprofit/go,danycoro/go,likesea/go,albertjin/golang-go,ImJasonH/go,jonathanmarvens/go,famorted/go,arschles/go-1,codestation/go,jonahglover/go,Pulgafree/go,ganboing/go-esx,b54898533/go,rachtsingh/gogojuice,baiyunping333/go,myPublicGit/go,vsdutka/go,uileyar/go,4ad/go,monetate/go,albertjin/golang-go,samanalysis/go,golang/go,codestation/go,xiaobodu/go,jacobhaven/go,qskycolor/go,Charlesdong/go-1,jasonxiong/go,greyhwndz/go,ErikDubbelboer/go,Young55555/go,g522342435/go,josharian/go,Young55555/go,vsekhar/elastic-go,jasonxiong/go,josharian/go,stormltf/go,theass/go,kakuhiroshi/go,vsdutka/go,greyhwndz/go,jameskumar/go,Samurais/go,b54898533/go,4ad/go,theass/go,CAFxX/go,zhangg/go,ericsnowcurrently/go,4ad/go,qskycolor/go,JioCloud/go,jacobxk/go,jacobxk/go,NichoZhang/go,donnel6809/go,cyberroadie/go,nakedgc/go,donnel6809/go,skatsuta/go,jonahglover/go,deft-code/go,josharian/go,xinhuang327/go,NichoZhang/go,kakuhiroshi/go,ifagnosticism/go,yuhengye/go,jdhenke/go,dpatel06/go,greyhwndz/go,xiuzhifu/go,g522342435/go,Endika/go,mdp/go,jdhenke/go,jefby/go,Pulgafree/go,Stan-Lin-/go,dterei/go,jasonxiong/go,fjballest/golang,deft-code/go,rentongzhang/go,gdey/go,zcwfeng/go,dpatel06/go,malvira/go,polyverse-security/go,xinhuang327/go,4ad/go,g522342435/go,gohin/go,lihuanghai/go,rachtsingh/gogojuice,leeric92/go,kevinc0825/go,sndnvaps/go,jefby/go,cloudrain21/go,Samurais/go,jonasi/go,willjunspecial/go,mgyenik/go,theass/go,NunoEdgarGub1/go,chinanjjohn2012/go,mwhudson/go,jdeng/go,Stan-Lin-/go,shines77/go,Jordanzuo/go,albertjin/golang-go,dterei/go,alex-zhang/go,Endika/go,zcwfeng/go,vsdutka/go,mikekap/go,cloudrain21/go,yangzhongj/go,yangzhongj/go,ifagnosticism/go,shines77/go,mdp/go,rachtsingh/gogojuice,wangfakang/go-1,ifagnosticism/go,ObjectiveJoe/go,cloudflare/go,binarycrusader/go,qskycolor/go,xujianhai/go,suncycheng/go,sinuos/go,jonathanmarvens/go,famorted/go,wangfakang/go-1,dterei/go,skatsuta/go,christopher-henderson/Go,ImJasonH/go,xinhuang327/go,honsiorovskyi/go,gdey/go,codestation/go,snowsnail/go,youprofit/go,danielsoro/go,rentongzhang/go,ifagnosticism/go,alash3al/go,skatsuta/go,jacobxk/go,odeke-em/go,greyhwndz/go,NunoEdgarGub1/go,dafyddcrosby/go,vanloswang/go,arschles/golang-go,danycoro/go,greyhwndz/go,mdp/go,miolini/go,rachtsingh/gogojuice,theif519/go,CarterTsai/go,monetate/go,fjballest/golang,deft-code/go,kevinc0825/go,WIZARD-CXY/go,donnel6809/go,jonasi/go,xinhuang327/go,jmptrader/go,zhangg/go,likesea/go,xiaobodu/go,yasarkunduz/go,parkdy/go,theass/go,vsdutka/go,danielsoro/go,Endika/go,XuYunnan/go,jameskumar/go,xiuzhifu/go,jameskumar/go,mgyenik/go,honsiorovskyi/go,danielsoro/go,josharian/go,mikekap/go,XuYunnan/go,xiuzhifu/go,cyberroadie/go,vishsingh/go,lihuanghai/go,jonasi/go,JioCloud/go,yasarkunduz/go,dafyddcrosby/go,honsiorovskyi/go,lihuanghai/go,mikekap/go,Endika/go,arschles/golang-go,vanloswang/go,monetate/go,gohin/go,Stan-Lin-/go,skatsuta/go,sinuos/go,linux-on-ibm-z/go,Stan-Lin-/go,wangfakang/go-1,theif519/go,mbrukman/go,zhangg/go,zcwfeng/go,cloudflare/go,albertjin/golang-go,devpadawan/go,jonasi/go,ifagnosticism/go,alex-zhang/go,stormltf/go,xiuzhifu/go,zouchao2010/go,stormltf/go,yasarkunduz/go,vanloswang/go,dterei/go,skotti/go,sosop/go,gohin/go,malvira/go,suncycheng/go,AALEKH/go,shazow/go,kim-racktop/go,famorted/go,springning/go,b54898533/go,dafyddcrosby/go,cloudflare/go,XuYunnan/go,stormltf/go,jacobhaven/go,jameskumar/go,willjunspecial/go,alex-zhang/go,andrewchambers/go,cookerzhu/go,gohin/go,Badredapple/go,JioCloud/go,roth1002/go,NichoZhang/go,akutz/go,akutz/go,AALEKH/go,sndnvaps/go,cookerzhu/go,Charlesdong/go-1,Pulgafree/go,jasonxiong/go,wangfakang/go-1,dpatel06/go,CAFxX/go,Young55555/go,vsekhar/elastic-go,samanalysis/go,ichu501/go,miolini/go,Endika/go,likesea/go,xinhuang327/go,snowsnail/go,likesea/go,baiyunping333/go,dafyddcrosby/go,shines77/go,binarycrusader/go,odeke-em/go,beealone/go,springning/go,akutz/go,Endika/go,binarycrusader/go,sosop/go,CAFxX/go,arschles/go-1,undergrowthlinear/go,honsiorovskyi/go,linux-on-ibm-z/go,Badredapple/go,arschles/golang-go,mikekap/go,CiscoCloud/go,dpatel06/go,danycoro/go,jdhenke/go,xujianhai/go,JioCloud/go,suncycheng/go,kakuhiroshi/go,vsekhar/elastic-go,GeorgiCodes/go,ImJasonH/go,sinuos/go,jdeng/go,ObjectiveJoe/go,qskycolor/go,danielsoro/go,slavau/go,donnel6809/go,kim-racktop/go,theif519/go,NichoZhang/go,yasarkunduz/go,joshsammut/go,miolini/go,alash3al/go,baiyunping333/go,WIZARD-CXY/go,vanloswang/go,ifagnosticism/go,CarterTsai/go,devpadawan/go,theif519/go,alex-zhang/go,lihuanghai/go,skatsuta/go,codestation/go,ImJasonH/go,goodlang/good,roth1002/go,monetate/go,yuhengye/go,codestation/go,fjballest/golang,willjunspecial/go,ObjectiveJoe/go,cookerzhu/go,slavau/go,mwhudson/go,NichoZhang/go,cainiaocome/go,jacobsa/go,nakedgc/go,cyberroadie/go,kakuhiroshi/go,zcwfeng/go,NunoEdgarGub1/go,cookerzhu/go,ganboing/go-esx,yuhengye/go,JioCloud/go,theif519/go,sndnvaps/go,theif519/go,shazow/go,alash3al/go,ericsnowcurrently/go,beealone/go,mdp/go,Jordanzuo/go,beealone/go,youprofit/go,AALEKH/go,miolini/go,akutz/go,odeke-em/go,cyberroadie/go,danycoro/go,kevinc0825/go,deft-code/go,vishsingh/go,mwhudson/go,cainiaocome/go,sosop/go,vishsingh/go,beealone/go,b54898533/go,ichu501/go,mbrukman/go,jacobxk/go,joshsammut/go,donnel6809/go,CodyGuo/go,samanalysis/go,odeke-em/go,Cofyc/go,shazow/go,WIZARD-CXY/go,Cofyc/go,ganboing/go-esx,jasonxiong/go,Endika/go,nawawi/go,gohin/go,cloudrain21/go,joshsammut/go,greyhwndz/go,zouchao2010/go,rentongzhang/go,constantine001/go,cookerzhu/go,kakuhiroshi/go,XuYunnan/go,skotti/go,NunoEdgarGub1/go,jefby/go,Badredapple/go,arschles/golang-go,jacobxk/go,zhangg/go,GeorgiCodes/go,ObjectiveJoe/go,binarycrusader/go,xiaobodu/go,mdp/go,undergrowthlinear/go,famorted/go,honsiorovskyi/go,CodyGuo/go,andrewchambers/go,alash3al/go,ganboing/go-esx,XuYunnan/go,gdey/go,CiscoCloud/go,CodyGuo/go,jacobhaven/go,theass/go,Young55555/go,nawawi/go,sinuos/go,undergrowthlinear/go,roth1002/go,miolini/go,danielsoro/go,zhangg/go,willjunspecial/go,leeric92/go,AALEKH/go,mikekap/go,wangfakang/go-1,myPublicGit/go,parkdy/go,yuhengye/go,skatsuta/go,ErikDubbelboer/go,jacobhaven/go,Pulgafree/go,leeric92/go,Jordanzuo/go,uileyar/go,theass/go,GeorgiCodes/go,alex-zhang/go,WIZARD-CXY/go,Young55555/go,Badredapple/go,jonasi/go,famorted/go,alash3al/go,springning/go,cloudrain21/go,zouchao2010/go,Young55555/go,ericsnowcurrently/go,ericsnowcurrently/go,undergrowthlinear/go,albertjin/golang-go,JioCloud/go,zcwfeng/go,springning/go,xujianhai/go,cyberroadie/go,andrewchambers/go,snowsnail/go,jacobsa/go,sosop/go,kim-racktop/go,springning/go,sinuos/go,CAFxX/go,skotti/go,vsdutka/go,beealone/go,kim-racktop/go,alash3al/go,miolini/go,baiyunping333/go,kevinc0825/go,4ad/go,andrewchambers/go,GeorgiCodes/go,vanloswang/go,baiyunping333/go,stormltf/go,jdeng/go,nawawi/go,likesea/go,myPublicGit/go,yangzhongj/go,likesea/go,xiuzhifu/go,ErikDubbelboer/go,arschles/golang-go,ImJasonH/go,xujianhai/go,derekmarcotte/go,jmptrader/go,shines77/go,vsekhar/elastic-go,jmptrader/go,myPublicGit/go,vsekhar/elastic-go,deft-code/go,nawawi/go,Jordanzuo/go,jdeng/go