Add Sign 297 "direction arrows" in documentation

Hello,
as far as we understood Roadmarkings Sign 297 "direction arrows` (Richtungspfeile) would be covered by roadmarkings with traffic_main_sign_type for example TYPE_PRESCRIBED_STRAIGHT_AHEAD or  TYPE_PRESCRIBED_RIGHT_TURN_AND_STRAIGHT_AHEAD etc.

If so, it would be good to add these Sign-Numbers in Proto-Files so text search runs more easily

Thanks