docs/JAMULUS_PROTOCOL.md: document raw audio option

Per review: mention the raw (uncompressed PCM) audio option alongside
OPUS/OPUS64 — how it's advertised (FS_RAW_AUDIO in
CLM_SERVER_FEATURES), disabled (--noraw), and how the Server tells it
apart from OPUS (packet-size comparison against the exact raw-PCM
byte count, since there's no distinct message type).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: jrd

docs/JAMULUS_PROTOCOL.md

@@ -283,7 +283,7 @@ A Directory is a Jamulus Server acting as a registry (implemented in `src/server
 
 The OPUS codec is used to compress the audio over the network and the packets are documented [here](https://datatracker.ietf.org/doc/html/rfc6716).
 
-Jamulus uses a custom OPUS encoder / decoder, giving some different frame sizes, but always uses a 48 kHz sample rate. OPUS and OPUS64 codecs are the only supported options currently.
+Jamulus uses a custom OPUS encoder / decoder, giving some different frame sizes, but always uses a 48 kHz sample rate. OPUS and OPUS64 codecs are the standard options; a Server can also accept raw (uncompressed 16-bit PCM) audio instead of OPUS-encoded audio, advertised via the `FS_RAW_AUDIO` bit in `CLM_SERVER_FEATURES` and disabled with the Server's `--noraw` option. A raw audio packet isn't tagged with a distinct message type — since the Server already learns the Client's chosen frame size via `NETW_TRANSPORT_PROPS`, it tells raw audio apart from OPUS by comparing the received packet's size against the exact byte count raw PCM would produce for that frame size and channel count.
 
 The packet size will vary based on: