Add extra information on AsEventListener attribute usage

Author: devojifr

event_dispatcher.rst

@@ -162,7 +162,9 @@ having to add any configuration in external files::
         }
     }
 
-You can add multiple ``#[AsEventListener()]`` attributes to configure different methods::
+You can add multiple ``#[AsEventListener()]`` attributes to configure different methods.
+In the below example, for ``foo`` event, ``onFoo`` method will be called
+implicitly if method attribute is not set::
 
     namespace App\EventListener;