[css-ui-4][editorial] Added 'user-select' property to changes sections

Obviously, this was missed earlier.

Author: SebastianZ

css-ui-4/Overview.bs

@@ -4290,6 +4290,9 @@ Changes from the <a href="https://www.w3.org/TR/2015/WD-css-ui-4-20150922/">22 S
 	the following normative changes have been made:
 
 	<ul>
+		<li>
+			Introduced the 'user-select' property.
+
 		<li>
 			The <css>caret-animation</css> property has been removed,
 			as there weren't any known use cases for it.
@@ -4313,6 +4316,9 @@ Changes from the <a href="https://www.w3.org/TR/2015/WD-css-ui-4-20150922/">22 S
 Changes from <a href="https://www.w3.org/TR/css-ui-3/">CSS UI Level 3</a></h3>
 
 	<ul>
+		<li>
+			Introduced the 'user-select' property.
+
 		<li>
 			Added the directional focus navigation properties 'nav-up', 'nav-down', 'nav-left', and 'nav-right'.