time: add a comment about how to use the Duration constants
time: add a comment about how to use the Duration constants

R=golang-dev, bradfitz, r, dsymonds
CC=golang-dev
http://codereview.appspot.com/5696078

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