Gruntfile: add note about duplicate copy of jqueryCheck code

Refs #13842

Gruntfile.js

@@ -32,6 +32,7 @@ module.exports = function (grunt) {
             ' * Copyright 2011-<%= grunt.template.today("yyyy") %> <%= pkg.author %>\n' +
             ' * Licensed under <%= pkg.license.type %> (<%= pkg.license.url %>)\n' +
             ' */\n',
+    // NOTE: This jqueryCheck code is duplicated in customizer.js; if making changes here, be sure to update the other copy too.
     jqueryCheck: 'if (typeof jQuery === \'undefined\') { throw new Error(\'Bootstrap\\\'s JavaScript requires jQuery\') }\n\n',
 
     // Task configuration.