[selectors-4] Add :focus-within to Overview

Author: LeaVerou

selectors-4/Overview.bs

@@ -286,6 +286,11 @@ Selectors Overview</h2>
 				<td>an E element that has user input focus
 				<td>[[#useraction-pseudos]]
 				<td>2
+			<tr>
+				<td><code>E:focus-within</code>
+				<td>an E element that has user input focus or contains an element that has input focus.
+				<td>[[#the-focus-within-pseudo]]
+				<td>4
 			<tr>
 				<td><code>E:focus-ring</code>
 				<td>an E element that has user input focus,