message: Explain why normalizing slashes on vendor config output
diff: @@ -298,6 +298,9 @@ fn sync(
  config.insert(
  merged_source_name.to_string(),
  VendorSource::Directory {
  + // Windows-flavour paths are valid here on Windows but Unix.
  + // This backslash normalization is for making output paths more
  + // cross-platform compatible.
  directory: opts.destination.to_string_lossy().replace("\\", "/"),
  },
  );
  