[css-position-4][editorial] Move document painting to a separate algo.

Author: tabatkins

css-position-4/Overview.bs

@@ -78,14 +78,19 @@ and contains the root element. Initially, the viewport is anchored
 with its top left corner at the canvas origin.
 
 <div algorithm>
-	To <dfn>paint a stacking context</dfn>
-	given a [=document=], [=element=], or [=box=] |root|,
+	To <dfn>paint a document</dfn>,
+	given a document |root|
 	and an infinite canvas |canvas|:
 
-	1. If |root| is a [=document=],
-		[=paint a stacking context=] given |root|'s root element
+	1. [=Paint a stacking context=] given |root|'s root element
 		 and |canvas|,
 		then return |canvas|.
+</div>
+
+<div algorithm>
+	To <dfn>paint a stacking context</dfn>
+	given an [=element=] or [=box=] |root|,
+	and an infinite canvas |canvas|:
 
 	1. If |root| is an [=element=],
 		[=paint a stacking context=] given |root|'s [=principal box=]