Add best practice note about version

Adding this info based on request by @nicolas-grekas in #27037 (symfony/symfony#27037)

Author: arderyp

components/phpunit_bridge.rst

@@ -33,6 +33,13 @@ Alternatively, you can clone the `<https://github.com/symfony/phpunit-bridge>`_
 
 .. include:: /components/require_autoload.rst.inc
 
+.. note::
+
+    The PHPUnit bridge is designed to work with all maintained versions of
+    Symfony components, even across different major versions of them. You should
+    always use its very latest stable major version to get the most accurate
+    deprecation report.
+
 Usage
 -----