Add info about EventManagerInterface::on() signature change.

en/appendices/6-0-migration-guide.rst

@@ -72,6 +72,14 @@ Some methods have also been renamed to better reflect their purpose. These are:
 * ``Cake\View\Helper\PaginatorHelper``
     - ``_numbers()`` has been renamed to ``buildNumbers()``
 
+Event
+-----
+
+- The signature of ``EventManagerInterface::on()`` has changed. The 2nd and 3rd
+  parameters have been swapped. Calls which pass an options array as the 2nd
+  argument will need to be updated to pass it as the 3rd argument instead or 
+  use named parameters.
+
 Datasource
 ----------
 

en/core-libraries/events.rst

@@ -412,8 +412,8 @@ for event listeners::
     $callback = [$this, 'doSomething'];
     $this->getEventManager()->on(
         'Order.afterPlace',
+        $callback,
         ['priority' => 2],
-        $callback
     );
 
     // Setting priority for a listener