doc: clarify zip behavior in go1.8.html
doc: clarify zip behavior in go1.8.html

Updates #17929

Change-Id: I03b8847384c6cd8c43fe8bf2d75cd26db7063b8b
Reviewed-on: https://go-review.googlesource.com/33762
Reviewed-by: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>

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