nixos/*: automatically convert option descriptions

conversions were done using https://github.com/pennae/nix-doc-munge
using (probably) rev f34e145 running

    nix-doc-munge nixos/**/*.nix
    nix-doc-munge --import nixos/**/*.nix

the tool ensures that only changes that could affect the generated
manual *but don't* are committed, other changes require manual review
and are discarded.

nixos/modules/services/networking/dnsdist.nix

@@ -11,7 +11,7 @@ let
 in {
   options = {
     services.dnsdist = {
-      enable = mkEnableOption "dnsdist domain name server";
+      enable = mkEnableOption (lib.mdDoc "dnsdist domain name server");
 
       listenAddress = mkOption {
         type = types.str;