ops: cross-reference authelia unit literals in both split files

nix/host-modules/swarm-authelia.nix

@@ -93,6 +93,8 @@ let
   # user, group and StateDirectory from the instance name
   # (`authelia` + `-<name>`). Naming them here rather than repeating the
   # literal keeps the generator unit below and the module in step.
+  # ./swarm-authelia-service.nix binds the same two names for `unit`;
+  # the two must agree.
   instance = "swarm";
   unitName = "authelia-${instance}";
   stateDir = "/var/lib/${unitName}";