[css-cascade-4] Add clarifying note about why encapuslation contexts have this cascading behavior.

Author: fantasai

css-cascade-4/Overview.bs

@@ -806,6 +806,11 @@ Cascade Sorting Order</h3>
 			[[DOM]] [=tree contexts=] are considered to be nested
 			in [=shadow-including tree order=].
 
+			Note: This effectively means that
+			[=normal=] declarations belonging to an [=encapsulation context=]
+			can set defaults that are easily overridden by the outer context,
+			while [=important=] declarations belonging to an [=encapsulation context=]
+			can enforce requirements that cannot be overridden by the outer context.
 <!--
 		<dt id='cascade-scope'>Scope
 		<dd>