[selectors] Add note clarifying that hover state propagates up from shadow trees.

Author: frivoal

selectors/Overview.bs

@@ -2152,6 +2152,9 @@ The pointer hover pseudo-class '':hover''</h3>
 	are still conforming.
 
 	The parent of an element that is '':hover'' is also in that state.
+	
+	Note: When an element that is '':hover'' is in a <a>shadow tree</a>,
+	the <a>host element</a> (and its parents) are also in the '':hover'' state. (See [[css-scoping-1]])
 
 	Host languages may define additional ways in which an element can match '':hover''.
 	For example, [[HTML5]] defines a <a element>label</a> element as <a href="http://www.whatwg.org/html/selectors.html#selector-hover">matching <code>:hover</code></a>