Add comment about disablePopups setting

lib/browser/guest-view-manager.js

@@ -225,6 +225,7 @@ const attachGuest = function (event, elementInstanceId, guestInstanceId, params)
     webPreferences.preloadURL = params.preload
   }
 
+  // Return null from native window.open if allowpopups is unset
   if (webPreferences.nativeWindowOpen === true && !params.allowpopups) {
     webPreferences.disablePopups = true
   }