os/exec: document that examples assume Unix
os/exec: document that examples assume Unix

Fixes #13693.

Change-Id: Ieb89d8a8874ee8c6b304de99dceb07193c26b60e
Reviewed-on: https://go-review.googlesource.com/18315
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/go,4ad/go,akutz/go,theif519/go,cyberroadie/go,sndnvaps/go,4ad/go,linux-on-ibm-z/go,Samurais/go,miolini/go,momchil-velikov/go,codestation/go,ganboing/go-esx,polyverse-security/go,Cofyc/go,miolini/go,mwhudson/go,CAFxX/go,theif519/go,golang/go,miolini/go,momchil-velikov/go,binarycrusader/go,josharian/go,Samurais/go,cyberroadie/go,mwhudson/go,CAFxX/go,suncycheng/go,suncycheng/go,miolini/go,akutz/go,miolini/go,ImJasonH/go,linux-on-ibm-z/go,josharian/go,cloudflare/go,Cofyc/go,mwhudson/go,mwhudson/go,sndnvaps/go,goodlang/good,derekmarcotte/go,mwhudson/go,cloudflare/go,derekmarcotte/go,ganboing/go-esx,ImJasonH/go,suncycheng/go,golang/go,goodlang/good,goodlang/good,polyverse-security/go,derekmarcotte/go,goodlang/good,ganboing/go-esx,vsekhar/elastic-go,polyverse-security/go,Endika/go,binarycrusader/go,vsekhar/elastic-go,vanloswang/go,cloudflare/go,Cofyc/go,Cofyc/go,binarycrusader/go,ImJasonH/go,cyberroadie/go,mwhudson/go,miolini/go,Endika/go,cloudflare/go,CAFxX/go,derekmarcotte/go,josharian/go,vanloswang/go,vsekhar/elastic-go,josharian/go,momchil-velikov/go,nawawi/go,Samurais/go,4ad/go,4ad/go,CAFxX/go,linux-on-ibm-z/go,binarycrusader/go,ganboing/go-esx,Cofyc/go,sndnvaps/go,golang/go,golang/go,ganboing/go-esx,goodlang/good,linux-on-ibm-z/go,linux-on-ibm-z/go,cyberroadie/go,4ad/go,binarycrusader/go,Cofyc/go,golang/go,cyberroadie/go,Cofyc/go,golang/go,nawawi/go,theif519/go,miolini/go,Endika/go,theif519/go,vanloswang/go,suncycheng/go,Cofyc/go,Samurais/go,CAFxX/go,vanloswang/go,akutz/go,vanloswang/go,cloudflare/go,cloudflare/go,ganboing/go-esx,vanloswang/go,vanloswang/go,polyverse-security/go,Samurais/go,sndnvaps/go,christopher-henderson/Go,vsekhar/elastic-go,codestation/go,theif519/go,cloudflare/go,polyverse-security/go,4ad/go,derekmarcotte/go,4ad/go,akutz/go,polyverse-security/go,ImJasonH/go,momchil-velikov/go,CAFxX/go,theif519/go,derekmarcotte/go,mwhudson/go,akutz/go,suncycheng/go,momchil-velikov/go,ganboing/go-esx,ImJasonH/go,derekmarcotte/go,cyberroadie/go,Samurais/go,derekmarcotte/go,sndnvaps/go,CAFxX/go,CAFxX/go,cloudflare/go,akutz/go,josharian/go,codestation/go,goodlang/good,golang/go,binarycrusader/go,ganboing/go-esx,momchil-velikov/go,goodlang/good,codestation/go,nawawi/go,vsekhar/elastic-go,codestation/go,momchil-velikov/go,codestation/go,Samurais/go,Endika/go,binarycrusader/go,josharian/go,vsekhar/elastic-go,goodlang/good,josharian/go,nawawi/go,theif519/go,cyberroadie/go,nawawi/go,polyverse-security/go,akutz/go,linux-on-ibm-z/go,polyverse-security/go,mwhudson/go,ImJasonH/go,vsekhar/elastic-go,sndnvaps/go,theif519/go,miolini/go,Endika/go,Endika/go,nawawi/go,codestation/go,binarycrusader/go,vsekhar/elastic-go,sndnvaps/go,linux-on-ibm-z/go,vanloswang/go,ImJasonH/go,Samurais/go,nawawi/go,Endika/go,momchil-velikov/go,suncycheng/go,Endika/go,suncycheng/go,suncycheng/go,4ad/go,nawawi/go,codestation/go,sndnvaps/go,josharian/go,linux-on-ibm-z/go,cyberroadie/go,akutz/go,ImJasonH/go