You don't do manual NAT. Since there isn't a way to make a gateway group the default gateway, OPNsense isn't really multi-WAN capable.
On top of that, my experience with open source projects is that they very seldom fix reported bugs and that they ignore patches when provided or find excuses to do nothing. So if I were to do anything with the documentation, I would expect it to be ignored, so I won't even try.