Don't abstract over UDP for this specific bit of empiracal evidence

Instead, just mention that it is one such example, since UDP is not the
only option for the underlying transport.

draft-ietf-quic-transport.md

@@ -2755,10 +2755,10 @@ detrimental effect on performance.
 A connection will time out if no packets are sent or received for a period
 longer than the time negotiated using the max_idle_timeout transport parameter;
 see {{termination}}.  However, state in middleboxes might time out earlier than
-that.  Though REQ-5 in {{?RFC4787}} recommends a 2 minute timeout interval,
-experience shows that sending packets every 30 seconds is necessary to prevent
-the majority of middleboxes from losing state for underlying-transport flows
-{{?GATEWAY=DOI.10.1145/1879141.1879174}}.
+that.  For example, though REQ-5 in {{?RFC4787}} recommends a 2 minute timeout
+interval for UDP mappings, experience shows that sending packets every 30
+seconds is necessary to prevent the majority of middleboxes from losing state
+for UDP flows {{?GATEWAY=DOI.10.1145/1879141.1879174}}.
 
 
 ## Immediate Close {#immediate-close}