Add versionadded directive

Co-authored-by: Oskar Stark <oskarstark@googlemail.com>

Author: t-richard

mailer.rst

@@ -1136,6 +1136,10 @@ disable asynchronous delivery.
             ],
         ]);
 
+.. versionadded:: 5.1
+
+    The `message_bus` option was introduced in Symfony 5.1.
+
 Adding Tags and Metadata to Emails
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