Explanation to desktopManagerHandlesLidAndPower

With this patch, systemd-inhibit outputs a descriptive message when
desktopManagerHandlesLidAndPower=true (the default).

Before the patch:

    $ systemd-inhibit
         Who: /nix/store/[...]-xsession [...] (UID 1000/cassou, PID 18561/systemd-inhibit)
        What: handle-power-key:handle-lid-switch
         Why: Unknown reason
        Mode: block

After the patch:

    $ systemd-inhibit
         Who: /nix/store/[...]-xsession [...] (UID 1000/cassou, PID 18561/systemd-inhibit)
        What: handle-power-key:handle-lid-switch
         Why: See NixOS configuration option 'services.xserver.displayManager.desktopManagerHandlesLidAndPower' for more information.
        Mode: block