Add a note about the scrollSource's ownerDocument changing. Fixes issue #2.

Author: theres-waldo

Overview.bs

@@ -376,6 +376,10 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 
     If this is not specified, the document element is used.
 
+    Note that the {{scrollSource}}'s {{ownerDocument}} may change over the lifetime
+    of the scroll timeline. This does not affect the scroll timeline; the timeline
+    continues to be driven by the scroll progress of the {{scrollSource}}.
+
 :   <dfn attribute for=ScrollTimeline>orientation</dfn>
 ::  Determines the direction of scrolling which triggers the activation and drives
     the progress of the trigger.