Clarify Ack-Eliciting Threshold of 0 and 1

fixes #307

Author: mirjak

draft-ietf-quic-ack-frequency.md

@@ -452,8 +452,14 @@ consecutive CE-marked packets are received or only non-CE-marked packet received
 the endpoint resumes sending acknowledgements based on the Ack-Eliciting
 Threshold or max_ack_delay. Therefore, CE-marking only triggers an immediate
 acknowledgement when there is a transition from non-CE-marked to CE-marked.
-
-Doing this maintains the peer's response time to congestion events, while also
+If the Ack-Eliciting Threshold is 0, every packet is anyway immediately
+acknowledged, no matter if marked with CE or not. If the Ack-Eliciting Threshold
+is 1, the default behavior as specified in RFC9000 applies, which recommends to
+immediately acknowledge all packets marked with CE (see
+{{Section 13.2.1 of QUIC-TRANSPORT}}).
+
+Always acknowledging the first marked CE packet immediately maintains
+the peer's response time to congestion events, while also
 reducing the ACK rate compared to {{Section 13.2.1 of QUIC-TRANSPORT}} during
 extreme congestion or when peers are using DCTCP {{?RFC8257}} or other
 congestion controllers (e.g. {{?I-D.ietf-tsvwg-aqm-dualq-coupled}}) that mark