gotest: document unit of time for elapsed()
gotest: document unit of time for elapsed()

R=golang-dev
CC=golang-dev
http://codereview.appspot.com/4543044

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

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