Add explanation of integer key usage

Currently, we consistently use integer keys in our object
definitions, with the human readable field name appended as a comment.
This unusual syntax has significant memory benefits for us, but could
use an explanation for people not familiar with this design.

index.bs

@@ -1514,7 +1514,12 @@ Appendix A: Messages {#appendix-a}
 
 The following messages are defined with [[CDDL]]. When
 integer keys are used, a comment is appended to the line to indicate
-the name of the field. Each root message (one that can be put into a
+the name of the field. This unusual syntax is consistently applied for object
+definitions in this specification due to it resulting in significantly smaller
+objects over-the-wire. Integer keys are used instead of object arrays to allow
+for easy indexing of optional fields.
+
+Each root message (one that can be put into a
 QUIC stream without being enclosed by another message) has a comment
 indicating the message type key.