[css-nav-1] Change the result of getSpatialNavigationContainer()

Relate to: w3c#3743 (comment)

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -479,9 +479,11 @@ the User Agent must run the steps described below:
 <div algorithm="getSpatialNavigationContainer steps">
 :   <dfn method for=Element lt="getSpatialNavigationContainer()">getSpatialNavigationContainer()</dfn>
 ::
-    1. Return the element if it is a <a>spatial navigation container</a>,
-        or the nearest ancestor of the element that is a <a>spatial navigation container</a>,
+    1. Return the nearest ancestor of the element that is a <a>spatial navigation container</a>,
         or the <a>document</a> if the nearest <a>spatial navigation container</a> is the viewport.
+    
+    Note: If the element is a <a>spatial navigation container</a>, {{getSpatialNavigationContainer()}} also returns
+    the nearest <a>spatial navigation container</a>, not the element itself.
 
 </div>