Modify the description more precisely

Author: Jihye Hong

css-nav-1/Overview.bs

@@ -1273,7 +1273,7 @@ the proper spatial navigation behavior.
         Else, the focus moves to the candidates in the viewport.
 
     <dt><dfn>focus</dfn>
-    <dd>The directional input moves the focus to the candidates(visible or not). The user cannot manually scroll the container.
+    <dd>The directional input moves the focus to all candidates inside a scroll container. The user cannot manually scroll the container.
 
     NOTE: If the 'focus' value is given to 'spatial-navigation-action', 'navnotarget' event occurs 
     when there isn’t any visible candidate in the spatial navigation container no matter what it can be scrolled more or not.
@@ -1286,8 +1286,9 @@ the proper spatial navigation behavior.
 </dl>
 
 <div class=example>
-In this example, there is an element which is out of the view within a scrollable element.
-Pressing the down arrow key moves the focus directly to it without scrolling mannually.
+In this example, a scrollable container is specified with `spatial-navigation-action: focus`.
+Inside the container, there is an element which is out of the view within a <a>scrollport</a>.
+Pressing the down arrow key moves the focus directly to it without scrolling manually.
 
     <figure>
         <img alt="" src="images/spatnav-action.png" style="width: 500px;"/>