go.tools/cmd/cover: better usage message
go.tools/cmd/cover: better usage message

R=adg, rsc
CC=golang-dev
https://codereview.appspot.com/10453044

tmc/mirror-go.tools,t-yuki/go.tools.old,tmc/mirror-go.tools,tmc/mirror-go.tools,t-yuki/go.tools.old,t-yuki/go.tools.old