QUIC: Add glossary entry for RIO

Reviewed-by: Neil Horman <nhorman@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from #23495)

doc/designs/quic-design/glossary.md

@@ -214,6 +214,9 @@ frame which was transmitted. See FIFM design document for details.
 **Remotely-initiated:** Refers to a QUIC stream which was initiated by the
 remote peer, rather than by the local application.
 
+**RIO:** Reactive I/O subsystem. Refers to the generic, non-QUIC specific parts
+of the asynchronous I/O handling code which the OpenSSL QUIC stack is built on.
+
 **RSTREAM:** Receive stream. Internal receive buffer management object used to
 store data which has been RX'd but not yet read by the application.