Clarify docs: return value of uniqueId

index.html

@@ -1278,6 +1278,7 @@ _("fabio").capitalize();
         <br />
         Generate a globally-unique id for client-side models or DOM elements
         that need one. If <b>prefix</b> is passed, the id will be appended to it.
+        Without <b>prefix</b>, returns an integer.
       </p>
       <pre>
 _.uniqueId('contact_');