Make the order more clear

Signed-off-by: Lei Jitang <[email protected]>

Author: coolljt0725

config.md

@@ -244,7 +244,7 @@ This specification uses the following terms:
             Implementations MUST generate an error if they encounter a unknown value while verifying or unpacking an image.
           </li>
           <li>
-            <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most.
+            <code>diff_ids</code> is an array of layer content hashes (<code>DiffIDs</code>), in order from bottom-most to top-most and bottom-most at index 0.
           </li>
         </ul>
 
@@ -265,7 +265,7 @@ This specification uses the following terms:
     </dt>
     <dd>
         <code>history</code> is an array of objects describing the history of each layer.
-	The array is ordered from bottom-most layer to top-most layer.
+	The array is ordered from bottom-most layer to top-most layer and bottom-most at index 0.
 	The object has the following fields.
 
         <ul>