Clarify MAX_SUBSCRIBE_ID (#611)

This is an attempt at clarifying MAX_SUBSCRIBE_ID, feel free to make
suggestions.

Fixes #547

Author: ianswett

draft-ietf-moq-transport.md

@@ -1840,9 +1840,9 @@ MAX_SUBSCRIBE_ID
 {: #moq-transport-max-subscribe-id format title="MOQT MAX_SUBSCRIBE_ID Message"}
 
 * Subscribe ID: The new Maximum Subscribe ID for the session. If a Subscribe ID
-equal or larger than this is received in any message, including SUBSCRIBE,
-the publisher MUST close the session with an error of 'Too Many Subscribes'.
-More on Subscribe ID in {{message-subscribe-req}}.
+{{message-subscribe-req}} equal or larger than this is received by the publisher
+that sent the MAX_SUBSCRIBE_ID, the publisher MUST close the session with an
+error of 'Too Many Subscribes'.
 
 ## SUBSCRIBES_BLOCKED {#message-subscribes-blocked}