net/http: update Response.Trailer doc
net/http: update Response.Trailer doc

I updated this in the previous commit (https://golang.org/cl/17931)
but noticed a typo. and it still wasn't great.

The Go 1.5 text was too brief to know how to use it:

    // Trailer maps trailer keys to values, in the same
    // format as the header.

Change-Id: I33c49b6a4a7a3596735a4cc7865ad625809da900
Reviewed-on: https://go-review.googlesource.com/17932
Reviewed-by: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>

vanloswang/go,miolini/go,suncycheng/go,deft-code/go,roth1002/go,cyberroadie/go,Samurais/go,cloudflare/go,kim-racktop/go,codestation/go,nawawi/go,Samurais/go,josharian/go,Endika/go,Cofyc/go,deft-code/go,momchil-velikov/go,cyberroadie/go,golang/go,sndnvaps/go,theif519/go,kim-racktop/go,ImJasonH/go,4ad/go,kim-racktop/go,roth1002/go,derekmarcotte/go,binarycrusader/go,codestation/go,goodlang/good,ganboing/go-esx,sndnvaps/go,vsekhar/elastic-go,josharian/go,binarycrusader/go,suncycheng/go,sndnvaps/go,Endika/go,theif519/go,CAFxX/go,vanloswang/go,theif519/go,codestation/go,vsekhar/elastic-go,binarycrusader/go,mwhudson/go,derekmarcotte/go,derekmarcotte/go,theif519/go,Endika/go,deft-code/go,4ad/go,vsekhar/elastic-go,nawawi/go,golang/go,roth1002/go,vanloswang/go,vsekhar/elastic-go,roth1002/go,linux-on-ibm-z/go,binarycrusader/go,goodlang/good,suncycheng/go,mwhudson/go,ImJasonH/go,cloudflare/go,vanloswang/go,suncycheng/go,vsekhar/elastic-go,nawawi/go,akutz/go,derekmarcotte/go,cyberroadie/go,kim-racktop/go,akutz/go,josharian/go,mwhudson/go,cloudflare/go,golang/go,deft-code/go,CAFxX/go,Endika/go,kim-racktop/go,linux-on-ibm-z/go,cyberroadie/go,cloudflare/go,mwhudson/go,cloudflare/go,Cofyc/go,cloudflare/go,akutz/go,deft-code/go,nawawi/go,vsekhar/elastic-go,goodlang/good,linux-on-ibm-z/go,cloudflare/go,Samurais/go,mwhudson/go,4ad/go,polyverse-security/go,josharian/go,codestation/go,linux-on-ibm-z/go,Samurais/go,Cofyc/go,vsekhar/elastic-go,sndnvaps/go,polyverse-security/go,theif519/go,miolini/go,momchil-velikov/go,codestation/go,josharian/go,vanloswang/go,suncycheng/go,golang/go,linux-on-ibm-z/go,akutz/go,Endika/go,4ad/go,miolini/go,derekmarcotte/go,binarycrusader/go,mwhudson/go,binarycrusader/go,CAFxX/go,CAFxX/go,derekmarcotte/go,sndnvaps/go,4ad/go,mwhudson/go,polyverse-security/go,binarycrusader/go,ganboing/go-esx,vsekhar/elastic-go,momchil-velikov/go,ganboing/go-esx,momchil-velikov/go,ImJasonH/go,ganboing/go-esx,Cofyc/go,josharian/go,golang/go,Cofyc/go,theif519/go,codestation/go,codestation/go,vanloswang/go,Samurais/go,derekmarcotte/go,theif519/go,miolini/go,Cofyc/go,Cofyc/go,polyverse-security/go,kim-racktop/go,sndnvaps/go,CAFxX/go,Cofyc/go,miolini/go,cyberroadie/go,CAFxX/go,akutz/go,roth1002/go,codestation/go,linux-on-ibm-z/go,CAFxX/go,golang/go,nawawi/go,cloudflare/go,Endika/go,josharian/go,goodlang/good,linux-on-ibm-z/go,4ad/go,ganboing/go-esx,ImJasonH/go,momchil-velikov/go,binarycrusader/go,nawawi/go,cyberroadie/go,polyverse-security/go,4ad/go,momchil-velikov/go,Samurais/go,Endika/go,derekmarcotte/go,Samurais/go,nawawi/go,deft-code/go,golang/go,deft-code/go,goodlang/good,mwhudson/go,ganboing/go-esx,deft-code/go,Samurais/go,vanloswang/go,ImJasonH/go,akutz/go,cyberroadie/go,kim-racktop/go,kim-racktop/go,sndnvaps/go,suncycheng/go,josharian/go,christopher-henderson/Go,momchil-velikov/go,goodlang/good,4ad/go,polyverse-security/go,CAFxX/go,ImJasonH/go,vanloswang/go,Endika/go,ganboing/go-esx,goodlang/good,miolini/go,ganboing/go-esx,linux-on-ibm-z/go,nawawi/go,miolini/go,roth1002/go,miolini/go,roth1002/go,suncycheng/go,golang/go,ImJasonH/go,ImJasonH/go,sndnvaps/go,momchil-velikov/go,akutz/go,theif519/go,akutz/go,goodlang/good,polyverse-security/go,polyverse-security/go,cyberroadie/go,roth1002/go,suncycheng/go