Update draft-ietf-quic-recovery.md

Co-authored-by: Martin Thomson <mt@lowentropy.net>

draft-ietf-quic-recovery.md

@@ -1134,7 +1134,7 @@ after periods of under-utilization, such as those proposed for TCP in
 ## Loss and Congestion Signals
 
 Loss detection and congestion control fundamentally involve the consumption of
-signals -- delay, loss, and ECN codepoints -- from unauthenticated entities.
+signals from unauthenticated entities.  Attackers can spoof or alter delay, loss, and ECN markings.
 On-path attackers can spoof or alter these signals. An attacker can cause
 endpoints to reduce their sending rate by dropping packets, altering path delay
 strategically, or by changing ECN codepoints.