network: Explain why a network does not include the domains.

networks.rkt

@@ -167,6 +167,11 @@
 (define update-function/c (-> state? any/c))
 
 ;;; A network is a mapping from its variables to its update functions.
+;;;
+;;; Note that the domains of the variables of the network are not part
+;;; of the network definition.  This is because the variables of some
+;;; networks may have infinite domains, which can be restricted in
+;;; multiple different ways.
 (define network? (hash/c variable? procedure?))
 
 ;;; Given a state s updates all the variables from xs.  This