The mapping is indeed a tad confusing. As a matter of fact I could not find a doc entry on the subject in ArduPilot documentation, either. We’ll fix it. Thanks for noticing! For now there’s a pretty good description here.
-C switch is SERIAL1
So, I suppose you can just change SERIAL1_PROTOCOL parameter to Frsky S-PORT in GCS while connected via Wi-Fi (-A udp:<ip>:<port>) and you’re good to go 