spec: clarify scope and re-use of iteration variables
spec: clarify scope and re-use of iteration variables

Fixes issue 7834.

LGTM=iant, rsc, r
R=r, rsc, iant, ken
CC=golang-codereviews
https://codereview.appspot.com/148940044

firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang