reflect: document tie-breaking in Select
reflect: document tie-breaking in Select

The exact words are taken from the spec.

Fixes some confusion on golang-nuts.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7353044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go