Group ID SHOULD increase with time (#749)

The text that came out of the Denver interim plus a short editorial
sentence.

Closes #727
Closes #587

Author: ianswett

draft-ietf-moq-transport.md

@@ -379,6 +379,12 @@ subscriber that does not want to receive the entire track can opt to receive onl
 the latest group(s).  The publisher then selectively transmits objects based on
 their group membership.  Groups can contain any number of objects.
 
+Within a track, the original publisher SHOULD produce Group IDs which increase
+with time. Subscribers to tracks which do not follow this requirement SHOULD NOT
+use range filters which span multiple groups in FETCH or SUBSCRIBE. SUBSCRIBE and
+FETCH delivery use Group Order, so a FETCH cannot deliver Groups out of order
+and a subscription could have unexpected delivery order if Group IDs do not increase
+with time.
 
 ## Track {#model-track}