net/http: explain the "1.1" in the default User-Agent

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/13263052

src/pkg/net/http/request.go

@@ -291,6 +291,11 @@ func valueOrDefault(value, def string) string {
 	return def
 }
 
+// NOTE: This is not intended to reflect the actual Go version being used.
+// It was changed from "Go http package" to "Go 1.1 package http" at the
+// time of the Go 1.1 release because the former User-Agent had ended up
+// on a blacklist for some intrusion detection systems.
+// See https://codereview.appspot.com/7532043.
 const defaultUserAgent = "Go 1.1 package http"
 
 // Write writes an HTTP/1.1 request -- header and body -- in wire format.