minor #18404 [Standards] Update code guidelines (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
----------

[Standards] Update code guidelines

I think coding standards could be updated to reflect [this discussion](symfony/symfony#50615 (comment)) 👍

Commits
-------

ed6bf81 [Standards] Update code guidelines

Author: javiereguiluz

contributing/code/standards.rst

@@ -190,6 +190,14 @@ Structure
 
 * Exception and error messages must start with a capital letter and finish with a dot ``.``;
 
+* Exception, error and deprecation messages containing a class name must
+  use ``get_debug_type()`` instead of ``::class`` to retrieve it:
+
+  .. code-block:: diff
+
+    - throw new \Exception(sprintf('Command "%s" failed.', $command::class));
+    + throw new \Exception(sprintf('Command "%s" failed.', get_debug_type($command)));
+
 * Do not use ``else``, ``elseif``, ``break`` after ``if`` and ``case`` conditions
   which return or throw something;