cmd/go: explain x... vs. x/... in help importpath Fixes issue 3110.
cmd/go: explain x... vs. x/... in help importpath
Fixes issue 3110.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/5696083

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