[scroll-animations-1] Add link to issue about source handling in constructor

Author: birtles

scroll-animations-1/Overview.bs

@@ -389,6 +389,9 @@ determined not by wall-clock time, but by the progress of scrolling in a
 
     1. Assign the {{ScrollTimeline/orientation}}, {{ScrollTimeline/start}}, {{ScrollTimeline/end}}, and {{ScrollTimeline/timeRange}} properties of |timeline| to the corresponding value from |options|.
 
+Issue(5202): The above steps need clarification, particularly with regards to
+handling of null values for |source|.
+
 </div>
 
 <div class="attributes">