Update index.bs

Co-authored-by: jgraham <[email protected]>

index.bs

@@ -717,6 +717,9 @@ A <dfn for=event>subscription</dfn> is a [=/struct=] consisting of a
 <dfn for=subscription>top-level traversable ids</dfn> (a [=/set=] of IDs of [=/top-level traversables=])
 and <dfn for=subscription>user context ids</dfn> (a [=/set=] of IDs of [=user contexts=]).
 
+Note: A [=subscription=] can have non-empty [=subscription/top-level traversable ids=],
+or non-empty [=subscription/user context ids=], but not both.
+
 A [=subscription=] |subscription| is <dfn for="subscription">global</dfn>
 if |subscription|'s [=subscription/top-level traversable ids=] is an empty set
 and |subscription|'s [=user context ids=] is an empty set.