spec: clarify lhs syntax for range and select
spec: clarify lhs syntax for range and select

Fixes issue 4653.

R=rsc, r, iant, ken, thakis
CC=golang-dev
https://codereview.appspot.com/7135058
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go