Merge branch '3.4' into 4.1

* 3.4:
  Update tags.rst
  Update service_subscribers_locators.rst

Author: javiereguiluz

service_container/service_subscribers_locators.rst

@@ -203,7 +203,7 @@ service type to a service.
 
     .. code-block:: yaml
 
-        // config/services.yaml
+        # config/services.yaml
         services:
             App\CommandBus:
                 tags:

service_container/tags.rst

@@ -389,10 +389,10 @@ To answer this, change the service declaration:
         // config/services.php
         $container->register(\Swift_SmtpTransport::class)
             ->addArgument('%mailer_host%')
-            ->addTag('app.mail_transport', ['alias' => 'foo']);
+            ->addTag('app.mail_transport', ['alias' => 'smtp']);
 
         $container->register(\Swift_SendmailTransport::class)
-            ->addTag('app.mail_transport', ['alias' => 'bar']);
+            ->addTag('app.mail_transport', ['alias' => 'sendmail']);
 
 .. tip::