Clarify the code example about comparing custom objects

Author: BonsaiDen

doc/instanceof.md

@@ -8,9 +8,14 @@ nearly as useless as the [typeof operator](#typeof).
 
     function Foo() {}
     function Bar() {}
-    Bar.prototype = Foo;
+    Bar.prototype = new Foo();
 
     new Bar() instanceof Bar; // true
+    new Bar() instanceof Foo; // true
+
+    // This just sets Bar.prototype to the function object Foo
+    // But not to an actual instance of Foo
+    Bar.prototype = Foo;
     new Bar() instanceof Foo; // false
 
 ### Using `instanceof` with native types