Add description to add_libpam-systemd.sh script

Author: ruzickap

scripts/ubuntu/add_libpam-systemd.sh

@@ -2,7 +2,8 @@
 
 # This script should fix problems with hanged ssh session when the Ubuntu server is rebooted
 # The same problem appear when using "vagrant halt"
-# I saw this issue in minimal installation for Xenial
+# I saw this issue in minimal installation for Ubuntu Xenial 16.04 (and probably older).
+# Later Ubuntu versions have this package installed by default.
 # Description: https://serverfault.com/questions/706475/ssh-sessions-hang-on-shutdown-reboot
 
 export DEBIAN_FRONTEND="noninteractive"