Clarify usage of processResults (#4706)

docs/_includes/options/data/ajax.html

@@ -92,7 +92,7 @@
   </h3>
 
   <p>
-    You can use the <code>ajax.processResults</code> option to modify the data returned from the server before passing it to Select2.
+    You can use the <code>ajax.processResults</code> option to modify the data returned from the server before passing it to Select2.  The data structure returned by <code>processResults</code> should match the format as specified above.
   </p>
 
 {% highlight js linenos %}