[css-position-4] Specify that scrollable containing block exists on the root scroller too. w3c#10861

Author: tabatkins

css-position-4/Overview.bs

@@ -370,6 +370,12 @@ the [=scrollable containing block=] generated by an element
 is considered a child of the element's normal [=containing block=],
 and a parent of any containing blocks generated by its descendants.
 
+This also applies to the [=root scroller=];
+a [=scrollable containing block=] that corresponds
+to the padding edges of the entire document
+is generated,
+and is considered a child of the [=initial containing block=].
+
 The [=scrollable containing block=] is not used by default,
 but certain CSS features
 (such as 'position-area')