Merge branch '5.1' into 5.2

* 5.1:
  [#14083] Revert mailer.headers option in 4.4 - 5.1

Author: wouterj

reference/configuration/framework.rst

@@ -183,8 +183,6 @@ Configuration
     * `sender`_
     * `recipients`_
 
-  * :ref:`headers <mailer-headers>`
-
 * `php_errors`_
 
   * `log`_
@@ -3077,16 +3075,6 @@ recipients set in the code.
             ]);
         };
 
-.. _mailer-headers:
-
-headers
-.......
-
-**type**: ``array``
-
-Headers to add to emails. key (``name`` attribute in xml format)
-is the header name and value the header value.
-
 workflows
 ~~~~~~~~~