go spec: clarify rules for append, scope rules for :=
go spec: clarify rules for append, scope rules for :=

Fixes issue 1936.
Fixes issue 1940.

R=rsc, r, iant, ken, iant
CC=golang-dev
http://codereview.appspot.com/4585049

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