Added a note about how to send emails with console commands

doc/providers/swiftmailer.rst

@@ -97,6 +97,22 @@ The Swiftmailer provider provides a ``mailer`` service::
         return new Response('Thank you for your feedback!', 201);
     });
 
+Usage in commands
+~~~~~~~~~~~~~~~~~
+
+The Swiftmailer provider sends the emails using the ``KernelEvents::TERMINATE``
+event, which is fired after the response has been sent. However, as this event
+isn't fired for console commands, your emails won't be sent.
+
+For that reason, if you send emails using a command console, make sure to
+flush the message spool by hand before ending the command execution. To do so,
+use the following code::
+
+    $app['swiftmailer.spooltransport']
+        ->getSpool()
+        ->flushQueue($app['swiftmailer.transport'])
+    ;
+
 Traits
 ------