net/http: document Hijacker and Flusher more
net/http: document Hijacker and Flusher more

Fixes #15312

Change-Id: I4fabef3f21081bc4b020069851b5c2504bc6b4d8
Reviewed-on: https://go-review.googlesource.com/22122
Reviewed-by: Emmanuel Odeke <277f8eaa8c7dfbc55c17bf5e30561c1a9c45c569@gmail.com>
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

linux-on-ibm-z/go,suncycheng/go,derekmarcotte/go,derekmarcotte/go,vsekhar/elastic-go,derekmarcotte/go,golang/go,goodlang/good,CAFxX/go,goodlang/good,CAFxX/go,Cofyc/go,CAFxX/go,akutz/go,golang/go,cloudflare/go,theif519/go,nawawi/go,vsekhar/elastic-go,linux-on-ibm-z/go,polyverse-security/go,golang/go,momchil-velikov/go,CAFxX/go,linux-on-ibm-z/go,4ad/go,Cofyc/go,ganboing/go-esx,codestation/go,Cofyc/go,cloudflare/go,momchil-velikov/go,ganboing/go-esx,vsekhar/elastic-go,akutz/go,derekmarcotte/go,linux-on-ibm-z/go,ganboing/go-esx,momchil-velikov/go,christopher-henderson/Go,akutz/go,suncycheng/go,vsekhar/elastic-go,polyverse-security/go,goodlang/good,josharian/go,linux-on-ibm-z/go,derekmarcotte/go,nawawi/go,polyverse-security/go,CAFxX/go,Cofyc/go,theif519/go,josharian/go,4ad/go,ganboing/go-esx,codestation/go,codestation/go,nawawi/go,polyverse-security/go,codestation/go,Cofyc/go,suncycheng/go,goodlang/good,polyverse-security/go,suncycheng/go,ImJasonH/go,golang/go,codestation/go,nawawi/go,Cofyc/go,josharian/go,golang/go,theif519/go,theif519/go,goodlang/good,momchil-velikov/go,theif519/go,momchil-velikov/go,ganboing/go-esx,ImJasonH/go,josharian/go,theif519/go,cloudflare/go,akutz/go,ImJasonH/go,nawawi/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,vsekhar/elastic-go,momchil-velikov/go,josharian/go,ImJasonH/go,vsekhar/elastic-go,CAFxX/go,josharian/go,suncycheng/go,nawawi/go,nawawi/go,cloudflare/go,ImJasonH/go,Cofyc/go,4ad/go,cloudflare/go,goodlang/good,ImJasonH/go,momchil-velikov/go,theif519/go,codestation/go,akutz/go,josharian/go,cloudflare/go,vsekhar/elastic-go,ImJasonH/go,4ad/go,linux-on-ibm-z/go,vsekhar/elastic-go,cloudflare/go,goodlang/good,akutz/go,ImJasonH/go,polyverse-security/go,golang/go,codestation/go,ganboing/go-esx,CAFxX/go,polyverse-security/go,polyverse-security/go,cloudflare/go,goodlang/good,4ad/go,nawawi/go,ganboing/go-esx,CAFxX/go,josharian/go,4ad/go,4ad/go,derekmarcotte/go,derekmarcotte/go,golang/go,Cofyc/go,suncycheng/go,suncycheng/go,theif519/go,momchil-velikov/go,golang/go,linux-on-ibm-z/go,suncycheng/go,akutz/go,akutz/go,4ad/go,ganboing/go-esx