minor symfony#15478 Added header removal notice (ThomasLandauer)

This PR was merged into the 4.4 branch.

Discussion
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Added header removal notice

Commits
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77f34c5 Added header removal notice

Author: javiereguiluz

mailer.rst

@@ -944,8 +944,9 @@ This can be configured by replacing the ``dsn`` configuration entry with a
             ],
         ]);
 
-By default the first transport is used. The other transports can be used by
-adding a text header ``X-Transport`` to an email::
+By default the first transport is used. The other transports can be selected by
+adding an ``X-Transport`` header (which Mailer will remove automatically from
+the final email)::
 
     // Send using first "main" transport ...
     $mailer->send($email);