spec: add example for continue to label
Make the break example slightly more interesting
Update issue 5725
Effective Go will be updated in a separate CL.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13368054
effective_go: add a discussion of labeled break and continue
Fixes issue 5725.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/13705044