[css-nav-1] Add note explaining why we have spatial-navigation-action

Author: frivoal

css-nav-1/Overview.bs

@@ -1426,6 +1426,13 @@ or of its nearest [=scroll container=] ancestor if it isn't.
         Note: The ''spatial-navigation-ation/scroll'' value is <a>at-risk</a>.
 </dl>
 
+Note: Earlier version of this specification did not offer a declarative way
+to opt into the behavior defined by ''spatial-navigation-action/focus'',
+and instead offered a cancellable event that would be fired before scrolling,
+so that authors could implement that behavior themselves.
+However, cancellable events related to scrolling can cause performance problems,
+so this event was removed and the 'spatial-navigation-action' property was introduced instead.
+
 <div class=example>
 In this example, a scrollable container is specified with <code highlight=css>spatial-navigation-action: focus</code>.
 Inside the container, there is an element which is out of the view within a <a>scrollport</a>.