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/virtualisation/qemu-vm.nix

@@ -584,7 +584,7 @@ in
       example = literalExpression ''
         import pkgs.path { system = "x86_64-darwin"; }
       '';
-      description = ''
+      description = lib.mdDoc ''
         pkgs set to use for the host-specific packages of the vm runner.
         Changing this to e.g. a Darwin package set allows running NixOS VMs on Darwin.
       '';