Immediate ACKs are good for PTO

Fixes #214

draft-ietf-quic-ack-frequency.md

@@ -451,6 +451,11 @@ will wait 'max_ack_delay', increasing the chances of a premature PTO.
 Therefore, if Ignore Order is enabled, the PTO MUST be larger than the peer's
 'max_ack_delay'.
 
+When sending PTO packets, one can either skip a sufficient number of packet numbers
+or include an IMMEDIATE_ACK frame to elicit an immediate acknowledgement. This
+avoids waiting for the ack delay for acknowledgements of PTO packets, reducing
+tail latency and allowing the sender to exclude the peer's 'max_ack_delay' from
+subsequent PTO calculations.
 
 # Determining Acknowledgement Frequency {#implementation}