defaultHardening: explain where @system-service is defined

pkgs/lib.nix

@@ -28,6 +28,7 @@ let self = {
       CapabilityBoundingSet = "";
       # @system-service whitelist and docker seccomp blacklist (except for "clone"
       # which is a core requirement for systemd services)
+      # @system-service is defined in src/shared/seccomp-util.c (systemd source)
       SystemCallFilter = [ "@system-service" "~add_key clone3 get_mempolicy kcmp keyctl mbind move_pages name_to_handle_at personality process_vm_readv process_vm_writev request_key set_mempolicy setns unshare userfaultfd" ];
       SystemCallArchitectures= "native";
   };