Update 3.3-di-changes.rst

Related to symfony#9676 (review)

Author: DonCallisto

service_container/3.3-di-changes.rst

@@ -567,6 +567,12 @@ Start by updating the service ids to class names:
     classes (e.g. ``Twig_Extensions_Extension_Intl`` instead of ``Twig\Extensions\IntlExtension``),
     you can't redefine the service as ``Twig_Extensions_Extension_Intl: ~`` and
     you must keep the original ``class`` parameter.
+    
+.. caution::
+
+    If a service is processed by `Compiler Pass`_ you could face a  "You have requested a non-existent service" error. 
+    To get rid of this, be sure that Compiler Pass is using ``findDefinition`` instead of ``getDefinition`` as the latter
+    won't take aliases into account when looking up for services.
 
 But, this change will break our app! The old service ids (e.g. ``app.github_notifier``)
 no longer exist. The simplest way to fix this is to find all your old service ids
@@ -778,3 +784,5 @@ can be autowired. The final configuration looks like this:
 You can now take advantage of the new features going forward.
 
 .. _`FrameworkExtension for 3.3.0`: https://github.com/symfony/symfony/blob/7938fdeceb03cc1df277a249cf3da70f0b50eb98/src/Symfony/Bundle/FrameworkBundle/DependencyInjection/FrameworkExtension.php#L247-L284
+
+.. _`Compiler Pass`: https://symfony.com/doc/current/service_container/compiler_passes.html