[css-position-3] Clarify non-impact of relpos. w3c#10522

Author: fantasai

css-position-3/Overview.bs

@@ -158,7 +158,8 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		The box is laid out as for ''static'',
 		then offset from the resulting position.
 		This offsetting is a purely visual effect,
-		and does not affect the size or position of any other box,
+		and, unless otherwise specified,
+		does not affect the size or position of any other non-descendant box
 		except insofar as it increases the [=scrollable overflow area=] of its ancestors.
 		This [=positioning scheme=] is called
 		<dfn export dfn for="" lt="relative position | relatively position | relatively-positioned | relatively positioned box" local-lt="relative|relatively">relative positioning</dfn>.