Hello Helge Kreutzmann, I'm skeptical how relevant it is to document how X and ssh works in the _su_ manpage, but either way since you're patching the manpage this is something you want to send upstream for review if you think it's worth persuing and my opinion doesn't count there.
As always to increase chances of a favourable review getting the administrative details right from the start is useful, so please see Documentation/howto-contribute.txt (You might also want to replace references to 'Debian 9' with 'shadow su implementation' or similar.) I'll add a note to util-linux.NEWS about DISPLAY and XAUTHORITY. Regards, Andreas Henriksson
