os/signal: windows is supported, update comment Fixes issue 5035.
os/signal: windows is supported, update comment
Fixes issue 5035.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7519045
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go