runtime/race: improve public documentation

Move the documentation from race.go to doc.go, because
race.go uses +build race, so it's not normally parsed by go doc.
Rephrase the documentation for end users, provide link to race
detector manual.
Fixes #5444.

R=golang-dev, minux.ma, adg, r
CC=golang-dev
https://golang.org/cl/9144050

src/pkg/runtime/race/doc.go

@@ -0,0 +1,9 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package race implements data race detection logic.
+// No public interface is provided.
+// For details about the race detector see
+// http://golang.org/doc/articles/race_detector.html
+package race

src/pkg/runtime/race/race.go

@@ -4,7 +4,6 @@
 
 // +build race,linux,amd64 race,darwin,amd64 race,windows,amd64
 
-// Package race provides low-level facilities for data race detection.
 package race
 
 /*