minor #9410 Better explain how to inject mailers different than the default one (javiereguiluz)

This PR was merged into the 4.0 branch.

Discussion
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Better explain how to inject mailers different than the default one

As reported in symfony/symfony#26444, this explanation could be updated to the latest Symfony services practices.

Commits
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dc287f4 Better explain how to inject mailers different than the default one

Author: javiereguiluz

reference/configuration/swiftmailer.rst

@@ -344,7 +344,7 @@ key (the default mailer is identified by the ``default_mailer`` option):
             ),
         ));
 
-Each mailer is registered as a service::
+Each mailer is registered automatically as a service with these IDs::
 
     // ...
 
@@ -362,3 +362,70 @@ Each mailer is registered as a service::
     When configuring multiple mailers, options must be placed under the
     appropriate mailer key of the configuration instead of directly under the
     ``swiftmailer`` key.
+
+When using :ref:`autowiring <services-autowire>` only the default mailer is
+injected when type-hinting some argument with the ``\Swift_Mailer`` class. If
+you need to inject a different mailer in some service, use any of these
+alternatives based on the :ref:`service binding <services-binding>` feature:
+
+.. configuration-block::
+
+    .. code-block:: yaml
+
+        # config/services.yaml
+        services:
+            _defaults:
+                bind:
+                    # this injects the second mailer when type-hinting constructor arguments with \Swift_Mailer
+                    \Swift_Mailer: '@swiftmailer.mailer.second_mailer'
+                    # this injects the second mailer when a service constructor argument is called $specialMailer
+                    $specialMailer: '@swiftmailer.mailer.second_mailer'
+
+            App\Some\Service:
+                # this injects the second mailer only for this argument of this service
+                $differentMailer: '@swiftmailer.mailer.second_mailer'
+
+            # ...
+
+    .. code-block:: xml
+
+        <!-- config/services.xml -->
+        <?xml version="1.0" encoding="UTF-8" ?>
+        <container xmlns="http://symfony.com/schema/dic/services"
+            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+            xsi:schemaLocation="http://symfony.com/schema/dic/services
+                http://symfony.com/schema/dic/services/services-1.0.xsd">
+
+            <services>
+                <defaults autowire="true" autoconfigure="true" public="false">
+                    <!-- this injects the second mailer when type-hinting constructor arguments with \Swift_Mailer -->
+                    <bind key="\Swift_Mailer">@swiftmailer.mailer.second_mailer</bind>
+                    <!-- this injects the second mailer when a service constructor argument is called $specialMailer -->
+                    <bind key="$specialMailer">@swiftmailer.mailer.second_mailer</bind>
+                </defaults>
+
+                <service id="App\Some\Service">
+                    <!-- this injects the second mailer only for this argument of this service -->
+                    <argument key="$differentMailer">@swiftmailer.mailer.second_mailer</argument>
+                </service>
+
+                <!-- ... -->
+            </services>
+        </container>
+
+    .. code-block:: php
+
+        // config/services.php
+        use App\Some\Service;
+        use Symfony\Component\DependencyInjection\Reference;
+        use Psr\Log\LoggerInterface;
+
+        $container->register(Service::class)
+            ->setPublic(true)
+            ->setBindings(array(
+                // this injects the second mailer when this service type-hints constructor arguments with \Swift_Mailer
+                \Swift_Mailer => '@swiftmailer.mailer.second_mailer',
+                // this injects the second mailer when this service has a constructor argument called $specialMailer
+                '$specialMailer' => '@swiftmailer.mailer.second_mailer',
+            ))
+        ;