Merge branch '5.4' into 6.2

* 5.4:
  add `OptionsResolverIntrospector`usage doc

Author: javiereguiluz

components/options_resolver.rst

@@ -929,3 +929,21 @@ method ``clearOptionsConfig()`` and call it periodically::
 
 That's it! You now have all the tools and knowledge needed to process
 options in your code.
+
+Getting More Insights
+~~~~~~~~~~~~~~~~~~~~~
+
+Use the ``OptionsResolverIntrospector`` to inspect the options definitions
+inside an ``OptionsResolver`` instance::
+
+   use Symfony\Component\OptionsResolver\Debug\OptionsResolverIntrospector;
+   use Symfony\Component\OptionsResolver\OptionsResolver;
+
+    $resolver = new OptionsResolver();
+    $resolver->setDefaults([
+        'host' => 'smtp.example.org',
+        'port' => 25,
+    ]);
+
+    $introspector = new OptionsResolverIntrospector($resolver);
+    $introspector->getDefault('host'); // Retrieves "smtp.example.org"