[scroll-animations-1] Add note about horizontal and vertical axis still using logical direction (w3c#8667)

Co-authored-by: fantasai <fantasai.bugs@inkedblade.net>

Author: flackr

scroll-animations-1/Overview.bs

@@ -214,6 +214,10 @@ spec: cssom-view-1; type: dfn;
 			then the [=scroll progress timeline=] is [=inactive timeline|inactive=].
 	</dl>
 
+	Note: Progress is in reference to the [=scroll origin=],
+	which can flip depending on [=writing mode=],
+	even when ''scroll-timeline-axis/horizontal'' or ''scroll-timeline-axis/vertical'' is specified.
+
 	References to the [=root element=] propagate to the document viewport
 	(which functions as its [=scroll container=]).