Background: add a comment for questionable GSettings usage

BackgroundSource relies on Background to watch its own settings.
Add a comment to explain that.

https://bugzilla.gnome.org/show_bug.cgi?id=746509

js/ui/background.js

@@ -527,6 +527,10 @@ const BackgroundSource = new Lang.Class({
         let file = null;
         let style;
 
+        // We don't watch changes to settings here,
+        // instead we rely on Background to watch those
+        // and emit 'changed' at the right time
+
         if (this._overrideImage != null) {
             file = Gio.File.new_for_path(this._overrideImage);
             style = GDesktopEnums.BackgroundStyle.ZOOM; // Hardcode