Fixes jashkenas#689, documenting the need to resort collections with …

…comparators.

index.html

@@ -1275,6 +1275,12 @@ <h2 id="Collection">Backbone.Collection</h2>
       "sort", which must return <tt>0</tt>, <tt>1</tt>, or <tt>-1</tt>,
       and is more similar to a <tt>sortBy</tt> &mdash; a much nicer API.
     </p>
+
+    <p class="warning">
+      Collections with comparator functions will not automatically re-sort if you 
+      later change model attributes, so you may wish to call <tt>sort</tt> after 
+      changing model attributes that would affect the order.
+    </p>
 
     <p id="Collection-sort">
       <b class="header">sort</b><code>collection.sort([options])</code>