Add a scroll anchoring suppression trigger for zero scroll offset

Author: chrishtr

css-scroll-anchoring-1/Overview.bs

@@ -229,6 +229,8 @@ These triggers are:
 	Note that this trigger applies regardless of whether the modified element is
 	on the path from the anchor node to the scrollable element.
 
+* The scroll offset of the scrollable element becoming zero.
+
 Note: Suppression triggers exist for compatibility with existing web content that has negative
 interactions with scroll anchoring due to shifting content in scroll event handlers.