[css-position-3] Clarify when scrolling/replication happens w3c#5332

(i.e. not when the fixedpos element is trapped by an ancestor element)

Author: fantasai

css-position-3/Overview.bs

@@ -268,8 +268,10 @@ Choosing A Positioning Scheme: 'position' property</h2>
 		except the box is positioned and sized relative to a [=fixed positioning containing block=]
 		(usually the [=viewport=] in [=continuous media=], or the [=page area=] in [=paged media=]).
 		The box’s position is fixed with respect to this reference rectangle:
+		when attached to the [=viewport=]
 		it does not move when the document is scrolled,
-		and is replicated on every page when it is paginated.
+		and when attached to the [=page area=]
+		is replicated on every page when the document is paginated.
 		This [=positioning scheme=] is called
 		<dfn export dfn for="" lt="fixed position | fixed-positioned | fixed-positioned box" local-lt="fixed">fixed positioning</dfn>
 		and is considered a subset of [=absolute positioning=].