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

R=adg, rsc
CC=golang-dev
https://golang.org/cl/10453044

syreclabs/go-tools,magiccao/tools,springning/tools,cmars/tools,syreclabs/go-tools,golang/tools,springning/tools,dmitris/tools,dmitris/tools,maddyonline/tools,syreclabs/go-tools,argandas/tools,sshetty10/tools,maddyonline/tools,cw2018/tools,maddyonline/tools,springning/tools,glasser/golang-tools,glasser/golang-tools,springning/tools,runcom/tools,jdhenke/tools,cw2018/tools,glasser/golang-tools,josharian/tools,argandas/tools,jdhenke/tools,syreclabs/go-tools,runcom/tools,cw2018/tools,matryer/tools,syreclabs/go-tools,sshetty10/tools,nicot/tools,runcom/tools,4ad/tools,jdhenke/tools,magiccao/tools,maddyonline/tools,nicot/tools,magiccao/tools,cmars/tools,syreclabs/go-tools,golang/tools,josharian/tools,4ad/tools,matryer/tools,matryer/tools,cw2018/tools,argandas/tools,josharian/tools,argandas/tools,matryer/tools,nicot/tools,golang/tools,runcom/tools,magiccao/tools,josharian/tools,glasser/golang-tools,dmitris/tools,cmars/tools,sshetty10/tools,jdhenke/tools,4ad/tools,sshetty10/tools,dmitris/tools,golang/tools,golang/tools,cmars/tools,nicot/tools