os: reference LookupEnv from the Getenv docs
os: reference LookupEnv from the Getenv docs

Fixes #16782

Change-Id: If54917bf5ca1588d8a6d443c3aa6e1d4ada6b620
Reviewed-on: https://go-review.googlesource.com/27322
Reviewed-by: anatoly techtonik <657222fa4408d5fb9c49497617f580f8c5974a3b@gmail.com>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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