[scroll-animations-1] Add link to issue about checking range of threshold

Author: birtles

scroll-animations-1/Overview.bs

@@ -522,6 +522,9 @@ dictionary ElementBasedOffset {
     of the target that is expected to be visible in  {{source}}'s [=scrollport=]
     at the intersection offset.
 
+Issue(5203): The range of the <code>threshold</code> member is not currently
+checked anywhere.
+
 </div>