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 issue 5444.

R=golang-dev, minux.ma, adg, r
CC=golang-dev
https://codereview.appspot.com/9144050