improve comment

src/components/structures/UserSettings.js

@@ -287,6 +287,7 @@ module.exports = React.createClass({
     _refreshMediaDevices: function(stream) {
         if (stream) {
             // kill stream so that we don't leave it lingering around with webcam enabled etc
+            // as here we called gUM to ask user for permission to their device names only
             stream.getTracks().forEach((track) => track.stop());
         }