net/http: document Response.Body Read+Close interaction with keep-alive

Fixes #5645

Change-Id: Ifb46d6faf7ac838792920f6fe00912947478e761
Reviewed-on: https://go-review.googlesource.com/1531Reviewed-by: David Symonds <dsymonds@golang.org>

src/net/http/response.go

@@ -48,7 +48,10 @@ 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-length body. It is the caller's responsibility to
-	// close Body.
+	// close Body. The default HTTP client's Transport does not
+	// attempt to reuse HTTP/1.0 or HTTP/1.1 TCP connections
+	// ("keep-alive") unless the Body is read to completion and is
+	// closed.
 	//
 	// The Body is automatically dechunked if the server replied
 	// with a "chunked" Transfer-Encoding.