[css-anchor-position-1] Use scrollable CB for position-area #10861

Author: fantasai

css-anchor-position-1/Overview.bs

@@ -632,9 +632,13 @@ what area of this [=position-area grid=] to lay out the positioned box in.
 
 Values other than ''position-area/none'' have the following additional effects:
 
+* The [=scrollable containing block=] is used in place of the [=local containing block=]
+    where applicable,
+    so that the entire [=scrollable overflow area=] (typically) is available
+    for positioning.
+* Any ''top/auto'' [=inset properties=] resolve to ''0''.
 * The ''align-self/normal'' value for the [=self-alignment properties=]
 	resolves to a corresponding value, see [[#position-area-alignment]].
-* Any ''top/auto'' [=inset properties=] resolve to ''0''.
 
 <h4 id="position-area-grid-resolution">
 Resolving the Position Area Grid</h4>