flag: document that Value.String must work on the zero value
flag: document that Value.String must work on the zero value

Otherwise flag.PrintDefaults will fail when it tries to determine
whether the default is the zero value.

Fixes #16694.

Change-Id: I253fbf11ffc0a9069fd48c2c3cf3074df53e3a03
Reviewed-on: https://go-review.googlesource.com/27003
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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