go/build: clarify Context.HasSubdir docs
go/build: clarify Context.HasSubdir docs

Fixes #17888

Change-Id: I7490b95a03b810a0f7ed1f07f37d7c7b3ac036be
Reviewed-on: https://go-review.googlesource.com/34240
Reviewed-by: Dmitri Shuralyov <0c2a7749d909c8353e59c3f9c6a4d19a5ce000c1@gmail.com>
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

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