net/http: clarify Request fields' client-vs-server semantics
net/http: clarify Request fields' client-vs-server semantics

Fixes Issue 7682

LGTM=adg
R=golang-codereviews, adg
CC=dsymonds, golang-codereviews, iant
https://codereview.appspot.com/83800043

pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go