compress/flate: clarify the behavior of Writer.Flush
compress/flate: clarify the behavior of Writer.Flush

Fixes #16068

Change-Id: I04e80a181c0b7356996f7a1158ea4895ff9e1e39
Reviewed-on: https://go-review.googlesource.com/28477
Run-TryBot: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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