Clarify timestamp tuple field and explain relay behavior

Author: afrind

draft-frindell-moq-chat.md

@@ -78,16 +78,21 @@ track is "chat".
 * user-id - the user ID
 * device-id - a unique identifier for each device for the user.  This allows
               the same user to join the chat from multiple devices
-* timestamp - the timestamp in seconds when the track started.  This allows a
-              stateless client to start publishing without accidentally
-              overwriting a previously sent group and object.
+* timestamp - the timestamp in seconds when the track started, encoded as a
+              string.  This allows a stateless client to start publishing
+              without accidentally overwriting a previously sent group and
+              object.
+              Note: the protocol will still function so long as each chat
+              client selects a monotonically increasing number for this field.
+              Using the common format described here could support future
+              functionality like pulling chat history.
 
 ## Joining the Chat
 
 To join the chat a participant sends a SUBSCRIBE_NAMESPACE message to the relay
-with a namespace prefix ("moq-chat", \<id\>) in order to receive all ANNOUNCE
-message from each participant in the chat and UNANNOUNCE messages when
-participants leave.
+with a namespace prefix ("moq-chat", \<id\>).  MoQ Relays track the current
+state of all announced namespaces and namespace subscriptions, and forward any
+matching ANNOUNCE or UNANNOUNCE messages to interested endpoints.
 
 The participant also sends an ANNOUNCE message for their chat track namespace.