spec: clarify embedding of interfaces
spec: clarify embedding of interfaces

Fixes issue 7886.

LGTM=iant, r, rsc
R=r, iant, rsc, ken
CC=golang-codereviews
https://codereview.appspot.com/149010043

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