minor symfony#11729 Updates the tip explaining how to set the base phpunit version (Patrick PawseyVale)

This PR was squashed before being merged into the 3.4 branch (closes symfony#11729).

Discussion
----------

Updates the tip explaining how to set the base phpunit version

Fixes symfony#11728

Commits
-------

25c0c63 Updates the tip explaining how to set the base phpunit version

Author: javiereguiluz

components/phpunit_bridge.rst

@@ -573,8 +573,14 @@ If you have installed the bridge through Composer, you can run it by calling e.g
 
 .. tip::
 
-    Set the ``SYMFONY_PHPUNIT_VERSION`` env var to e.g. ``5.5`` to change the
-    base version of PHPUnit to ``5.5`` instead of the default ``5.3``.
+    It's possible to to change the base version of PHPUnit to e.g. ``5.5``
+    instead of the default ``5.3`` by setting the ``SYMFONY_PHPUNIT_VERSION``
+    env var in the ``phpunit.xml.dist`` file e.g.
+    ``<server name="SYMFONY_PHPUNIT_VERSION" value="5.5" />``. This is the
+    preferred method as it can be committed to your version control repository.
+
+    It's also possible set ``SYMFONY_PHPUNIT_VERSION`` as a real env var
+    (not in a :doc:`dotenv </components/dotenv>` file).
 
 .. tip::