cmd/vet: diagnose unreachable code

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7493048
cmd/gc: fix line number for 'missing return' in closure

R=ken2
CC=golang-dev
https://codereview.appspot.com/7838048
doc/go1.1.html: return requirements

R=golang-dev, rsc, jeremyjackins, gri
CC=golang-dev
https://codereview.appspot.com/7838045