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://golang.org/cl/87550045

src/pkg/net/http/response.go

@@ -47,7 +47,8 @@ type Response struct {
 	//
 	// The http Client and Transport guarantee that Body is always
 	// non-nil, even on responses without a body or responses with
-	// a zero-lengthed body.
+	// a zero-length body. It is the caller's responsibility to
+	// close Body.
 	//
 	// The Body is automatically dechunked if the server replied
 	// with a "chunked" Transfer-Encoding.