Specify which document's document element is used as the scrollSource if one is not specified explicitly. Fixes issue #5.

Author: theres-waldo

Overview.bs

@@ -376,7 +376,9 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 ::  The scrollable element whose scrolling triggers the activation and drives the
     progress of the timeline.
 
-    If this is not specified, the document element is used.
+    If this is not specified, the document element of the [=active document=] for the
+    current [=browsing context=], as determined at the time the {{ScrollTimeline}} is
+    constructed, 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