nixos: remove all uses of lib.mdDoc

these changes were generated with nixq 0.0.2, by running

  nixq ">> lib.mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> mdDoc[remove] Argument[keep]" --batchmode nixos/**.nix
  nixq ">> Inherit >> mdDoc[remove]" --batchmode nixos/**.nix

two mentions of the mdDoc function remain in nixos/, both of which
are inside of comments.

Since lib.mdDoc is already defined as just id, this commit is a no-op as
far as Nix (and the built manual) is concerned.

nixos/modules/misc/documentation/test.nix

@@ -30,7 +30,7 @@ let
     specialArgs.someArg.myModule = { lib, ... }: {
       options.foobar = lib.mkOption {
         type = lib.types.str;
-        description = lib.mdDoc "The foobar option was added via specialArgs";
+        description = "The foobar option was added via specialArgs";
         default = "qux";
       };
     };