Add note about covert channel feasability

Author: Hernqvist

index.bs

@@ -11760,6 +11760,11 @@ covert channel between two cooperating sites.
 One site could transmit information by intentionally causing audio glitches
 (by causing very high CPU usage, for example) while the other site
 could detect these glitches.
+
+Note: This covert channel depends on specific system characteristics.
+It typically requires a shared linearization point (such as an OS or browser
+audio mixer) and callbacks that are effectively synchronous from that point,
+without intermediate buffering that would flatten load spikes.
 <h5 id="AudioPlaybackStats-mitigations">Mitigations</h5>
 To inhibit the usage of such a covert channel, the API implements these
 mitigations.