minor #9689 Update swiftmailer.rst (ArchieDoe, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 2.7 branch instead (closes #9689).

Discussion
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Update swiftmailer.rst

Adding "command" parameter to sendmail transport.

Commits
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dfe33e3 Minor fixes
c9adc2f Update swiftmailer.rst

Author: javiereguiluz

reference/configuration/swiftmailer.rst

@@ -22,6 +22,7 @@ Configuration
 * `transport`_
 * `username`_
 * `password`_
+* `command`_
 * `host`_
 * `port`_
 * `timeout`_
@@ -77,6 +78,13 @@ password
 
 The password when using ``smtp`` as the transport.
 
+command
+~~~~~~~~
+
+**type**: ``string`` **default**: ``/usr/sbin/sendmail -bs``
+
+Command to be executed by ``sendmail`` transport.
+
 host
 ~~~~