nixos/networking: Enhance hostId description (#94800)

Co-authored-by: Jörg Thalheim <Mic92@users.noreply.github.com>

nixos/modules/tasks/network-interfaces.nix

@@ -408,6 +408,9 @@ in
         (this derives it from the machine-id that systemd generates) or
 
         <literal>head -c4 /dev/urandom | od -A none -t x4</literal>
+
+        The primary use case is to ensure when using ZFS that a pool isn't imported
+        accidentally on a wrong machine.
       '';
     };