os: add examples of environment functions
os: add examples of environment functions

For #16360.

Change-Id: Iaa3548704786018eacec530f7a907b976fa532fe
Reviewed-on: https://go-review.googlesource.com/27443
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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