os/exec: add example for CommandContext
os/exec: add example for CommandContext

Updates #16360

Change-Id: I0e0afe7a89f2ebcb3e5bbc345f77a605d3afc398
Reviewed-on: https://go-review.googlesource.com/30103
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

ImJasonH/go,ImJasonH/go,goodlang/good,cloudflare/go,goodlang/good,momchil-velikov/go,derekmarcotte/go,ImJasonH/go,josharian/go,ImJasonH/go,nawawi/go,nawawi/go,goodlang/good,akutz/go,linux-on-ibm-z/go,goodlang/good,linux-on-ibm-z/go,nawawi/go,cloudflare/go,suncycheng/go,CAFxX/go,vsekhar/elastic-go,nawawi/go,nawawi/go,Cofyc/go,linux-on-ibm-z/go,linux-on-ibm-z/go,linux-on-ibm-z/go,suncycheng/go,golang/go,suncycheng/go,cloudflare/go,vsekhar/elastic-go,akutz/go,josharian/go,cloudflare/go,josharian/go,momchil-velikov/go,goodlang/good,josharian/go,derekmarcotte/go,cloudflare/go,polyverse-security/go,akutz/go,Cofyc/go,vsekhar/elastic-go,codestation/go,golang/go,nawawi/go,cloudflare/go,Cofyc/go,derekmarcotte/go,momchil-velikov/go,derekmarcotte/go,nawawi/go,golang/go,polyverse-security/go,CAFxX/go,akutz/go,josharian/go,linux-on-ibm-z/go,cloudflare/go,polyverse-security/go,codestation/go,suncycheng/go,goodlang/good,CAFxX/go,suncycheng/go,vsekhar/elastic-go,momchil-velikov/go,Cofyc/go,codestation/go,polyverse-security/go,derekmarcotte/go,golang/go,Cofyc/go,goodlang/good,golang/go,ImJasonH/go,polyverse-security/go,goodlang/good,Cofyc/go,codestation/go,codestation/go,golang/go,akutz/go,derekmarcotte/go,vsekhar/elastic-go,codestation/go,nawawi/go,codestation/go,suncycheng/go,suncycheng/go,derekmarcotte/go,vsekhar/elastic-go,linux-on-ibm-z/go,momchil-velikov/go,akutz/go,momchil-velikov/go,polyverse-security/go,vsekhar/elastic-go,ImJasonH/go,CAFxX/go,linux-on-ibm-z/go,Cofyc/go,momchil-velikov/go,suncycheng/go,josharian/go,codestation/go,derekmarcotte/go,polyverse-security/go,christopher-henderson/Go,CAFxX/go,golang/go,josharian/go,momchil-velikov/go,josharian/go,Cofyc/go,CAFxX/go,CAFxX/go,akutz/go,CAFxX/go,ImJasonH/go,golang/go,vsekhar/elastic-go,cloudflare/go,polyverse-security/go,ImJasonH/go,akutz/go