inventory: add docs for available specialArgs in service submodules

lib/modules/inventory/flake-module.nix

@@ -38,7 +38,34 @@
         (pkgs.nixosOptionsDoc {
           options =
             (self.clanLib.evalService {
-              modules = [ { _docs_rendering = true; } ];
+              modules = [
+                { _docs_rendering = true; }
+                {
+                  options."#specialArgs" = lib.mkOption {
+                    description = ''
+                      Adidtional arguments passed to the module. Often referred to as `specialArgs`.
+
+                      ```nix
+                      {
+                        # root directory of the clan
+                        directory,
+                        # clanLib - The clan library functions
+                        clanLib,
+                        # exports from all services
+                        exports,
+                        ...
+                      }:
+                      {
+                        _class = "clan.service";
+                        manifest.name = "example-service";
+
+                        # ... elided
+                      }
+                      ```
+                    '';
+                  };
+                }
+              ];
               prefix = [ ];
             }).options;
           warningsAreErrors = true;