nixos/modules: use defaultText where applicable

Primarily to fix rendering of these default values in the manual but
it's also nice to avoid having to eval these things just to build the
manual.

nixos/modules/services/web-apps/quassel-webserver.nix

@@ -31,6 +31,8 @@ in {
       };
       pkg = mkOption {
         default = pkgs.quassel-webserver;
+        defaultText = "pkgs.quassel-webserver";
+        type = types.package;
         description = "The quassel-webserver package";
       };
       quasselCoreHost = mkOption {