Deferred.progress: Document it can take more than 2 arguments

More conform with the .fail and .done doc

Closes gh-933

Author: Tom Delmas

entries/deferred.progress.xml

@@ -14,7 +14,7 @@
       <type name="Function"/>
       <type name="Array"/>
       <desc>
-        Optional additional function, or array of functions, to be called when the Deferred generates progress notifications.
+        Optional additional functions, or arrays of functions, to be called when the Deferred generates progress notifications.
       </desc>
     </argument>
   </signature>