net/http: document ResponseWriter.WriteHeader more
net/http: document ResponseWriter.WriteHeader more

Change-Id: I65209b90ed7c56d4c751b3e4b3ce1de52dae368c
Reviewed-on: https://go-review.googlesource.com/82635
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

cloudflare/go,akutz/go,golang/go,akutz/go,golang/go,Cofyc/go,CAFxX/go,codestation/go,golang/go,CAFxX/go,golang/go,golang/go,nawawi/go,nawawi/go,codestation/go,akutz/go,nawawi/go,josharian/go,josharian/go,nawawi/go,cloudflare/go,josharian/go,josharian/go,codestation/go,cloudflare/go,codestation/go,josharian/go,CAFxX/go,codestation/go,codestation/go,nawawi/go,akutz/go,Cofyc/go,CAFxX/go,akutz/go,Cofyc/go,golang/go,golang/go,christopher-henderson/Go,cloudflare/go,akutz/go,golang/go,akutz/go,CAFxX/go,nawawi/go,akutz/go,Cofyc/go,josharian/go,CAFxX/go,Cofyc/go,josharian/go,Cofyc/go,josharian/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,Cofyc/go,CAFxX/go,cloudflare/go,codestation/go,codestation/go,Cofyc/go,nawawi/go,cloudflare/go