doc: mention the install locations for godoc, cover, and vet
doc: mention the install locations for godoc, cover, and vet

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/13893043

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang