[native_assets_cli] Better fix for dartdoc (#951)

Author: dcharkes

pkgs/native_assets_cli/dartdoc_options.yaml

@@ -5,6 +5,7 @@
 /// This library should not be used by `build.dart` scripts.
 ///
 /// Only `package:native_assets_builder` should use this.
+/// @nodoc
 library native_assets_cli_internal;
 
 export 'src/model/asset.dart';