feature #5724 Describe configuration behaviour with multiple mailers (xelan)

This PR was squashed before being merged into the 2.3 branch (closes #5724).

Discussion
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Describe configuration behaviour with multiple mailers

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | yes
| Applies to    | all
| Fixed tickets | symfony/swiftmailer-bundle#111

This PR describes the behaviour when multiple mailers are configured.

Commits
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887e8ba Describe configuration behaviour with multiple mailers

Author: wouterj

reference/configuration/swiftmailer.rst

@@ -303,3 +303,10 @@ Each mailer is registered as a service::
 
     // returns the second mailer
     $container->get('swiftmailer.mailer.second_mailer');
+
+.. caution::
+
+    When configuring multiple mailers, options must be placed under the appropriate
+    mailer key of the configuration. Options placed directly under the ``swiftmailer``
+    key will not be applied to all mailers, but will configure a mailer named
+    ``default`` instead.