clarify that Bundle and AbstractBundle are different

Please see https://stackoverflow.com/questions/74704768/invalidconfigurationexception-with-custom-bundle-in-symfony6-2 for the details.

Author: Andreas-Schoenefeldt

bundles/configuration.rst

@@ -319,13 +319,17 @@ In your extension, you can load this and dynamically set its arguments::
             // ... now use the flat $config array
         }
 
-Using the Bundle Class
-----------------------
+Using the AbstractBundle Class
+------------------------------
 
 .. versionadded:: 6.1
 
     The ``AbstractBundle`` class was introduced in Symfony 6.1.
 
+.. caution::
+
+    This is an alternative implementation to the above mentioned Bundle class. You can use one of these approaches but not both at the same time.
+
 Instead of creating an extension and configuration class, you can also
 extend :class:`Symfony\\Component\\HttpKernel\\Bundle\\AbstractBundle` to
 add this logic to the bundle class directly::