[css-position-3] Describe relation of abspos to scrollable overflow, cross-reference layout section.

Author: fantasai

css-position-3/Overview.bs

@@ -243,9 +243,12 @@ Choosing A Positioning Scheme: 'position' property</h2>
 
 		Instead, the box is positioned and sized solely in reference to
 		its [=absolute positioning containing block=],
-		as modified by the box's [=inset properties=].
-		It can overlap in-flow siblings
-		or other [=absolutely positioned=] elements.
+		as modified by the box's [=inset properties=],
+		see [[#abspos-layout]].
+		It can overlap [=in-flow=] content
+		or other [=absolutely positioned=] elements,
+		and is included in the [=scrollable overflow area=]
+		of the box that generates is [=containing block=].
 		This [=positioning scheme=] is called
 		<dfn export dfn lt="absolute position|absolutely position|absolutely-positioned|absolutely positioned box" local-lt="absolute|absolutely">absolute positioning</dfn>.