Add comment explaining notification markup

js/gratipay/notification.js

@@ -6,6 +6,15 @@ Gratipay.notification = function(text, type, timeout, closeCallback) {
     var type = type || 'notice';
     var timeout = timeout || (type == 'error' ? 10000 : 5000);
     var id = Math.random().toString(36).substring(2, 100);
+
+    // We want the notification to be fixed at the top of the page even while
+    // scrolling, without interfering with the heading that's already at the
+    // top of the page when scrolled the whole way up (logo, sign in, etc.). So
+    // what we do is create two notification divs. One sits behind and pushes
+    // the rest of the page content down, so that the header remains visible.
+    // The other sits in front, and is the one the user actually interacts
+    // with. It stays at the top of the viewport even while scrolling.
+
     var placeholder = ['div', {'class': 'notification notification-' + type}, ['div', text]];
     var dialog = ['div', {'class': 'notification notification-' + type, 'id': 'notification-'+id},
                                                                                     ['div', text]];