net/http: clarify Response.Body Close responsibility
net/http: clarify Response.Body Close responsibility

Per TODO email in my inbox.

LGTM=rsc
R=golang-codereviews, rsc
CC=adg, dsymonds, golang-codereviews, r
https://codereview.appspot.com/87550045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang