doc: add comment to example about 2xx status codes

doc/api/http.md

@@ -2236,6 +2236,8 @@ http.get('http://nodejs.org/dist/index.json', (res) => {
   const contentType = res.headers['content-type'];
 
   let error;
+  // Any 2xx status code signals a successful response but
+  // here we're only checking for 200.
   if (statusCode !== 200) {
     error = new Error('Request Failed.\n' +
                       `Status Code: ${statusCode}`);