[DOC] Add better docs for shadow passwords

Submitted by brt..@..il.com

Assigned to Sean Davis @bluesabre

Link to original bug (#15713)

Description

Operating system: openSUSE Leap 15.1 Screensaver versions: 0.1.5 and 0.1.6

Screensaver itself was working nicely but for a long time I couldn't have gotten screen locking to work. No prompt for password appears and the system was immediately unlocked after returning from screensaver animation. Application is generated with -enable-locking flag, which autogen confirms in it's summary. Running daemon in debugging mode gave an error for ConsoleKit not being found and a warning about xfce4-screensaver-dialog not being able to get password. After digging a bit into ConsoleKit i just enabled systemd (-with-systemd flag), which needed systemd-devel package. Only then I was able to spot an error telling me to chown and chmod xfce4-screensaver-dialog, unless I totally missed it before. It took me half a day to get it to work. Can we improve docs or add a faq about this? There's no visible indicator that something's wrong, just non-functional locking.