[e] (0) Clarify the WebVTT rendering rules.

Affected topics: Video Text Tracks

git-svn-id: http://svn.whatwg.org/webapps@6839 340c8d12-0b0e-0410-8428-c7bf67bfef74

source

@@ -104616,6 +104616,9 @@ iframe:not([seamless]) { border: 2px inset; }
 
       </dl>
 
+      <p class="note">These are not final positions, they are merely
+      temporary positions used to calculate box dimensions below.</p>
+
      </li>
 
      <li><p>Let <var title="">left</var> be '<var