[css-scroll-snap-2] Snap events bubble at the document. (#10377)

This specifies the resolution on
#10173 making snap events
bubble when fired at the document.

Co-authored-by: David Awogbemila <awogbemila@chromium.org>
Co-authored-by: Tab Atkins Jr <jackalmage@gmail.com>

css-scroll-snap-2/Overview.bs

@@ -585,7 +585,8 @@ it is the snap position
 that the snap container will eventually snap to
 when the scrolling operation ends.
 
-{{SnapEvent}}s do not bubble and are not cancellable.
+{{SnapEvent}}s do not bubble except when the event target is the {{Document}}.
+{{SnapEvent}}s are not cancellable.
 
 
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