Clarify 3/4 value computed value for background-position

Author: fantasai

css3-background/Overview.html

@@ -1008,7 +1008,9 @@ <h3 id=the-background-position><span class=secno>3.6. </span>The &lsquo;<a
 
      <td>If one or two values are specified, for a &lt;length&gt; the
       absolute length, otherwise a percentage. If three or four values are
-      specified, two pairs of a keyword plus a length or percentage.
+      specified, the horizontal origin keyword and length or percentage
+      offset followed by the vertical origin keyword and length or percentage
+      offset.
   </table>
 
   <p>If background images have been specified, this property specifies their