spec: clarify a couple of issues about channels
spec: clarify a couple of issues about channels

1. They are FIFOs.
2. They are safe for concurrent access.

Fixes issue 5911.

R=golang-dev, dvyukov, adg
CC=golang-dev
https://codereview.appspot.com/11549043

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