os/exec: add examples for CombinedOutput, StdinPipe, StderrPipe
os/exec: add examples for CombinedOutput, StdinPipe, StderrPipe

Updates #16360.

Adds examples for:
+ CombinedOutput
+ StdinPipe
+ StderrPipe

Change-Id: I19293e64b34ed9268da00e0519173a73bfbc2c10
Reviewed-on: https://go-review.googlesource.com/29150
Run-TryBot: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

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