go spec: attempt at clarifying language for "append"
go spec: attempt at clarifying language for "append"

Specifically, fix a wrong comment.

Fixes issue 1717.

R=r, rsc
CC=golang-dev
http://codereview.appspot.com/4445050
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go