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/services/home-automation/zigbee2mqtt.nix

@@ -18,12 +18,12 @@ in
   ];
 
   options.services.zigbee2mqtt = {
-    enable = mkEnableOption (lib.mdDoc "zigbee2mqtt service");
+    enable = mkEnableOption "zigbee2mqtt service";
 
     package = mkPackageOption pkgs "zigbee2mqtt" { };
 
     dataDir = mkOption {
-      description = lib.mdDoc "Zigbee2mqtt data directory";
+      description = "Zigbee2mqtt data directory";
       default = "/var/lib/zigbee2mqtt";
       type = types.path;
     };
@@ -40,7 +40,7 @@ in
           };
         }
       '';
-      description = lib.mdDoc ''
+      description = ''
         Your {file}`configuration.yaml` as a Nix attribute set.
         Check the [documentation](https://www.zigbee2mqtt.io/information/configuration.html)
         for possible options.