nixos/timesyncd: further document services.timesyncd.servers

Running systemd-timesyncd with an empty list of timeservers to sync from
does not work.

In case an empty list is configured here, systemd will fall back to its
compiled-in defaults, which NixOS sets to `{0..4}.nixos.pool.ntp.org`,
as per https://github.com/systemd/systemd/blob/main/docs/DISTRO_PORTING.md#ntp-pool

This has caused some confusion. Explicitly document this, and describe
how to disable timesyncd.

nixos/modules/system/boot/timesyncd.nix

@@ -21,6 +21,9 @@ with lib;
         type = types.listOf types.str;
         description = lib.mdDoc ''
           The set of NTP servers from which to synchronise.
+          Note if this is set to an empty list, the defaults systemd itself is
+          compiled with ({0..4}.nixos.pool.ntp.org) apply,
+          In case you want to disable timesyncd altogether, use the `enable` option.
         '';
       };
       extraConfig = mkOption {