doc/go1.2.txt: three-index slices are going into 1.2
doc/go1.2.txt: three-index slices are going into 1.2

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/12931044

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