Incorporate feedback

Adopt Will's more concise language and add Alan's clarification about
the edge case of tracks which haven't had any objects published to them
yet.

1. moq-wg#638 (comment)
2. moq-wg#638 (comment)

Author: englishm

draft-ietf-moq-transport.md

@@ -1527,9 +1527,8 @@ The following values are used:
   * For Subscribes with Filter Type AbsoluteStart or AbsoluteRange, this is equal to the StartObject field of the Subscribe message
 * Preceding Group Offset: A field in the Joining Fetch message indicating the relative offset from the start of the Subscribe
 
-Note: If a relay does not yet have LatestGroup and LatestObject for a given track, it can choose to either forward both the Subscribe and
-the Joining Fetch upstream or to wait until the Joining Fetch can be resolved locally. In either case, the Resolved Subscribe Start values
-for a Joining Fetch MUST correspond to the Subscribe within the same session so the Fetch and Subscribe can be contiguous and non-overlapping.
+The Resolved Subscribe Start values for a Joining Fetch MUST correspond to the referenced Subscribe within the same session so that the Objects delivered across the Fetch and Subscribe are contiguous and non-overlapping.
+If a relay answers the referenced Subscribe with a `SUBSCRIBE_OK` that has ContentExists set to 0, it MUST respond to the Joining Fetch with a `FETCH_ERROR`.
 
 Using that information and the following algorithm, these values are computed: