go spec: for map types, mention indexing operations          (like we do for arrays and slices).
go spec: for map types, mention indexing operations
         (like we do for arrays and slices).

Suggested by mathieu.lonjaret@gmail.com .

R=r, rsc, iant
CC=golang-dev
http://codereview.appspot.com/4442074

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