tweak banner of autogenerated CommonJS module

grunt/bs-commonjs-generator.js

@@ -2,7 +2,7 @@
 var fs = require('fs');
 var path = require('path');
 
-var COMMONJS_BANNER = '// This file is generated. You can require() it in a CommonJS environment.\n';
+var COMMONJS_BANNER = '// This file is autogenerated via the `commonjs` Grunt task. You can require() this file in a CommonJS environment.\n';
 
 module.exports = function generateCommonJSModule(grunt, srcFiles, destFilepath) {
   var destDir = path.dirname(destFilepath);