nixos/*: fix docbook deprecation notices

mostly no rendering changes except in buildkite, which used markdown
where docbook was expected without marking up its markdown.

nixos/modules/services/continuous-integration/buildkite-agents.nix

@@ -9,7 +9,7 @@ let
     inherit name;
     value = mkOption {
       default = null;
-      inherit description;
+      description = lib.mdDoc description;
       type = types.nullOr types.lines;
     } // (if example == null then {} else { inherit example; });
   };