[IMP] website: improve documentation of user custom files

See https://github.com/odoo/odoo/pull/29999

addons/website/static/src/js/user_custom_javascript.js

@@ -1 +1,24 @@
-// Here, you can add your custom JavaScript code.
+//
+// This file is meant to regroup your javascript code. You can either copy/past
+// any code that should be executed on each page loading or write your own
+// taking advantage of the Odoo framework to create new behaviors or modify
+// existing ones. For example, doing this will greet any visitor with a 'Hello,
+// world !' message in a popup:
+//
+/*
+odoo.define('website.user_custom_code', function (require) {
+'use strict';
+
+var Dialog = require('web.Dialog');
+var sAnimations = require('website.content.snippets.animation');
+
+sAnimations.registry.HelloWorldPopup = sAnimations.Class.extend({
+    selector: '#wrapwrap',
+
+    start: function () {
+        Dialog.alert(this, "Hello, world!");
+        return this._super.apply(this, arguments);
+    },
+})
+});
+*/