Multiple PHP class per file not allowed

Since `AnnotationFileLoader` from `Routing` component parses and loads only one class per file while ignoring others class if declared in the same file, it is good to let know people about this behaviour to not be surprised.

Author: ybenhssaien

routing.rst

@@ -46,6 +46,10 @@ following configuration file:
 This configuration tells Symfony to look for routes defined as annotations in
 any PHP class stored in the ``src/Controller/`` directory.
 
+.. caution::
+
+    If you define multiple PHP class per file, only the first declared class can be loaded, try to define one PHP class per file instead, visit `PSR-4 <https://www.php-fig.org/psr/psr-4/>`_ for more details.
+
 Suppose you want to define a route for the ``/blog`` URL in your application. To
 do so, create a :doc:`controller class </controller>` like the following::