Elaborate on potential use-cases for the upward message kinds.

roadmap/implementers-guide/src/messaging.md

@@ -36,7 +36,9 @@ The weight that processing of the dispatchables can consume is limited by a prec
 that some dispatchables will be left for later blocks. To make the dispatching more fair, the queues are processed turn-by-turn
 in a round robin fashion.
 
-Other kinds of upward messages can be introduced in the future as well. Most likely the HRMP channel management.
+Other kinds of upward messages can be introduced in the future as well. Potential candidates are channel management for
+horizontal message passing (XCMP and HRMP, both are to be described later), new validation code signalling, or other
+requests to the relay chain.
 
 ## Horizontal Message Passing