minor #15868 [Mailer] Explaining In-Reply-To and References header (ThomasLandauer)

This PR was squashed before being merged into the 4.4 branch.

Discussion
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[Mailer] Explaining In-Reply-To and References header

* ... as announced in #13869 ;-)
* Other change: "autoresponder" seems to be the more common term than "autoreplier": https://en.wikipedia.org/wiki/Autoresponder

Closes #13869

Commits
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2344303 [Mailer] Explaining In-Reply-To and References header

Author: javiereguiluz

mailer.rst

@@ -323,10 +323,13 @@ header, etc.) but most of the times you'll set text headers::
 
     $email = (new Email())
         ->getHeaders()
-            // this header tells auto-repliers ("email holiday mode") to not
+            // this non-standard header tells compliant autoresponders ("email holiday mode") to not
             // reply to this message because it's an automated email
             ->addTextHeader('X-Auto-Response-Suppress', 'OOF, DR, RN, NRN, AutoReply')
 
+            // use an array if you want to add a header with multiple values
+            // (for example in the "References" or "In-Reply-To" header)
+            ->addIdHeader('References', ['123@example.com', '456@example.com']);
         // ...
     ;