[css-cascade-4] Connect context handling better with shadow DOM terminology.

Author: fantasai

css-cascade-4/Overview.bs

@@ -795,12 +795,17 @@ Cascade Sorting Order</h3>
 			A document language can provide for blending declarations sourced
 			from different <dfn>encapsulation contexts</dfn>,
 			such as the nested [=tree contexts=] of [=shadow trees=] in the [[!DOM]].
+
 			When comparing two declarations
 			that are sourced from different [=encapsulation contexts=],
 			then for [=normal=] rules
 			the declaration from the outer context wins,
 			and for [=important=] rules
 			the declaration from the inner context wins.
+			For this purpose,
+			[[DOM]] [=tree contexts=] are considered to be nested
+			in [=shadow-including tree order=].
+
 <!--
 		<dt id='cascade-scope'>Scope
 		<dd>