[css3-values] Added entry to changelog about the attr() change regarding undefined namespace prefixes.

Author: tabatkins

css3-values/Overview.html

@@ -2251,6 +2251,9 @@ <h2 class=no-num id=changes>Changes</h2>
   <ul>
    <li>Corrected <code>wqname</code> in the ‘<a href="#attr"><code
     class=css>attr()</code></a>’ syntax to <code>qname</code>
+
+   <li>Made undefined namespace prefixes in ‘<a href="#attr"><code
+    class=css>attr()</code></a>’ invalidate the function.
   </ul>
 
   <h2 class=no-num id=references>References</h2>

css3-values/Overview.src.html

@@ -1742,6 +1742,7 @@ <h2 class="no-num" id="changes">Changes</h2>
 
   <ul>
     <li>Corrected <code>wqname</code> in the ''attr()'' syntax to <code>qname</code>
+    <li>Made undefined namespace prefixes in ''attr()'' invalidate the function.
   </ul>
 
 <h2 class="no-num" id="references">References</h2>