Document sortBy stability

index.html

@@ -631,6 +631,8 @@ <h2 id="collections">Collection Functions (Arrays or Objects)</h2>
         Returns a sorted copy of <b>list</b>, ranked in ascending order by the
         results of running each value through <b>iterator</b>. Iterator may
         also be the string name of the property to sort by (eg. <tt>length</tt>).
+        The sort algorithm used is stable. That is, order is preserved for equal
+        iterator results.
       </p>
       <pre>
 _.sortBy([1, 2, 3, 4, 5, 6], function(num){ return Math.sin(num); });