Log Message:
pam-pwauth_suid: add an additional detail to the MESSAGE file

Right now, a user installing xscreensaver with the "pam" option will
see two messages about how to configure it, one of which comes from
this package. This needs to be disentangled properly, but there are
some broader questions that also apply to gnome-screensaver and mate-
screensaver, so for now, add a bit more detail here just in case. (This
relates to, but doesn't particularly address, PR pkg/50622.)