Add note explaining why RTT is in PROBE message

Not all browsers/QUIC libraries expose RTT stats yet, so the server
needs to report it. May be deprecated once universally available.

https://claude.ai/code/session_01LCkQcr69mLdMHo9RQSL8xG

Author: claude

draft-lcurley-moq-lite.md

@@ -589,6 +589,8 @@ A value of 0 means unknown.
 The smoothed round-trip time in milliseconds, as defined in {{!RFC9002}}.
 A value of 0 means unknown.
 
+> NOTE: RTT is included in the PROBE message because not all QUIC implementations and browser WebTransport APIs expose RTT statistics directly. This field may be deprecated once RTT is universally available via the underlying transport API.
+
 ## GOAWAY
 A GOAWAY message is sent to initiate a graceful session shutdown with an optional redirect.