Cross-reference CLI and XML configuration settings

[kkmuffme]

xml: https://docs.phpunit.de/en/11.1/configuration.html#the-phpunit-element
cli: https://docs.phpunit.de/en/11.1/textui.html#configuration

It would be helpful if each contained a cross-reference to the other one (e.g. failOnWarning <-> --fail-on-warning)
This would also allow people to directly see which flags are available only for xml/cli and don't have a corresponding one.