[release-branch.go1.1] runtime/race: improve public documentation

««« CL 9144050 / d29da2ced72b
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
»»»

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/9387043