doc/go_spec.html: clarification about insertion during map iteration
doc/go_spec.html: clarification about insertion during map iteration

R=mdempsky, iant, r, gri, rsc, ken
CC=golang-dev
https://codereview.appspot.com/7100046

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