New phpunit10 options.

Closes #3444792

docs/jobs/phpunit.md

@@ -98,3 +98,9 @@ phpunit:
   variables:
     SYMFONY_DEPRECATIONS_HELPER: "ignoreFile=./.local-deprecation-ignore.txt"
 ```
+
+Finally, and if you are using a Drupal core version that comes with PHPUnit 10 or above (this would be Drupal 11 or above), you can also set the flag `--fail-on-deprecation` in the `_PHPUNIT_EXTRA` variable. This is only needed if you are running the tests in non-concurrent mode (see above). The code would look like this:
+```
+variables:
+  _PHPUNIT_EXTRA: '--fail-on-deprecation'
+```