sync/atomic: make BUG comment godoc-compatible
sync/atomic: make BUG comment godoc-compatible

go/doc wants to see text after BUG(uid): on the same line
in order to treat it as an official bug comment.

R=gri
CC=golang-dev
http://codereview.appspot.com/4250043
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go