[css-position-3] Clarify that the viewport is *usually* the fixpos containing block. #5332

Author: tabatkins

css-position-3/Overview.bs

@@ -266,7 +266,7 @@ Choosing A Positioning Scheme: 'position' property</h2>
 	<dd>
 		Same as ''absolute'',
 		except the box is positioned and sized relative to a [=fixed positioning containing block=]
-		(the [=viewport=] in [=continuous media=], or the [=page area=] in [=paged media=]).
+		(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:
 		it does not move when the document is scrolled,
 		and is replicated on every page when it is paginated.