flag: improve comment for calling String with zero value
flag: improve comment for calling String with zero value

Update #16694.

Change-Id: Id6be1535d8a146b3dac3bee429ce407a51272032
Reviewed-on: https://go-review.googlesource.com/27634
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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