docs: add output comment in Symbols article

Author: radlinskii

1-js/04-object-basics/08-symbol/article.md

@@ -169,7 +169,7 @@ for (let key in user) alert(key); // name, age (no symbols)
 */!*
 
 // the direct access by the symbol works
-alert( "Direct: " + user[id] );
+alert( "Direct: " + user[id] ); // Direct: 123
 ```
 
 [Object.keys(user)](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/keys) also ignores them. That's a part of the general "hiding symbolic properties" principle. If another script or a library loops over our object, it won't unexpectedly access a symbolic property.