Update documentation for `--filter`

[sebastianbergmann]

This is straight from the docs and I was using it when directly interfacing with the phpunit executable. I've added multiple --filter options to target specific test in a class and it worked. Maybe I'm misunderstanding something then?

The relevant docs section: https://docs.phpunit.de/en/12.0/textui.html#selection

EDIT: Ok, it seems the 2nd filter just overrides the first one. So how do I target specific test?

EDIT2: Mkey, most likely I need then run it on specific file with a filter for single method. Thanks!

Originally posted by @furai in #6219