go spec: clarify promotion rules for methods/fields of anonymous fields
go spec: clarify promotion rules for methods/fields of anonymous fields

Fixes issue 3635.

R=rsc, r, iant, kevlar, iant
CC=golang-dev
http://codereview.appspot.com/6217045

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang