compress/flate: document HuffmanOnly
compress/flate: document HuffmanOnly

Fixes #16489

Change-Id: I13e2ed6de59102f977566de637d8d09b4e541980
Reviewed-on: https://go-review.googlesource.com/25200
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>
Run-TryBot: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,CAFxX/go,Cofyc/go,nawawi/go,suncycheng/go,suncycheng/go,CAFxX/go,nawawi/go,nawawi/go,linux-on-ibm-z/go,vsekhar/elastic-go,cloudflare/go,akutz/go,4ad/go,golang/go,nawawi/go,suncycheng/go,linux-on-ibm-z/go,momchil-velikov/go,CAFxX/go,nawawi/go,4ad/go,goodlang/good,akutz/go,ImJasonH/go,goodlang/good,Cofyc/go,derekmarcotte/go,cloudflare/go,polyverse-security/go,golang/go,codestation/go,CAFxX/go,momchil-velikov/go,cloudflare/go,akutz/go,Cofyc/go,goodlang/good,derekmarcotte/go,codestation/go,cloudflare/go,momchil-velikov/go,josharian/go,4ad/go,ImJasonH/go,suncycheng/go,derekmarcotte/go,goodlang/good,vsekhar/elastic-go,golang/go,golang/go,momchil-velikov/go,nawawi/go,vsekhar/elastic-go,vsekhar/elastic-go,ImJasonH/go,goodlang/good,derekmarcotte/go,Cofyc/go,codestation/go,linux-on-ibm-z/go,josharian/go,ImJasonH/go,akutz/go,CAFxX/go,akutz/go,josharian/go,codestation/go,derekmarcotte/go,ImJasonH/go,nawawi/go,cloudflare/go,ImJasonH/go,polyverse-security/go,vsekhar/elastic-go,momchil-velikov/go,polyverse-security/go,polyverse-security/go,akutz/go,nawawi/go,linux-on-ibm-z/go,josharian/go,suncycheng/go,josharian/go,ImJasonH/go,golang/go,Cofyc/go,polyverse-security/go,Cofyc/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,CAFxX/go,josharian/go,cloudflare/go,polyverse-security/go,derekmarcotte/go,Cofyc/go,vsekhar/elastic-go,momchil-velikov/go,goodlang/good,suncycheng/go,cloudflare/go,CAFxX/go,4ad/go,4ad/go,suncycheng/go,christopher-henderson/Go,linux-on-ibm-z/go,Cofyc/go,4ad/go,cloudflare/go,polyverse-security/go,goodlang/good,josharian/go,akutz/go,codestation/go,ImJasonH/go,4ad/go,josharian/go,codestation/go,momchil-velikov/go,akutz/go,vsekhar/elastic-go,4ad/go,golang/go,golang/go,goodlang/good,polyverse-security/go,momchil-velikov/go,derekmarcotte/go,CAFxX/go,linux-on-ibm-z/go,vsekhar/elastic-go,linux-on-ibm-z/go,suncycheng/go,codestation/go