[css-animations][bikeshed] Add missing example 4

Author: Sylvain Galineau

css-animations/Overview.bs

@@ -585,6 +585,12 @@ The 'animation' shorthand property</h3>
 	output in at least the cases necessary to distinguish an 'animation-name' that could 
 	be a value of another property, and may be output in additional cases. 
 
+	<div class="example">
+	For example, a value parsed from ''animation: 3s none backwards'' (where 'animation-fill-mode' is ''none'' 
+	and 'animation-name' is ''backwards'') must not be serialized as ''animation: 3s backwards'' (where 
+	'animation-fill-mode' is ''backwards'' and 'animation-name' is ''none''). 
+	</div>
+
 <h2 id="events">
 Animation Events</h2>