man: add a note about $XDG_SEAT and $XDG_VTNR to pam_systemd(8)

Issue #6499 requests that a mention that those varibles can be set in the
environment is added. But the man page already says that. There isn't much
detail, but a man page does not need to and in this case should not include
all the details. Instead a note is added that those vars can be derived from
$DISPLAY.

Closes #6499.