[Mime][Mailer] add DraftEmail documentation

Author: kbond

mailer.rst

@@ -1266,6 +1266,46 @@ The following transports only support tags:
 
 * OhMySMTP
 
+Draft Emails
+------------
+
+.. versionadded:: 6.1
+
+    ``Symfony\Component\Mime\DraftEmail`` was introduced in ``symfony/mime`` 6.1.
+
+:class:`Symfony\\Component\\Mime\\DraftEmail` is a special instance of
+:class:`Symfony\\Component\\Mime\\Email`. Its purpose is to build up an email
+(with body, attachments, etc) and make available to download as an ``.eml`` with
+the ``X-Unsent`` header. Many email clients can open these files and interpret
+them as *draft emails*. You can use these to create advanced ``mailto:`` links.
+
+Here's an example of making one available to download::
+
+    use Symfony\Component\HttpFoundation\Response;
+    use Symfony\Component\HttpFoundation\ResponseHeaderBag;
+    use Symfony\Component\Mime\DraftEmail;
+
+    $content = (new DraftEmail())
+        ->html($twig->render(/* ... */))
+        ->attach(/* ... */)
+        ->toString()
+    ;
+
+    $response = new Response($message->toString());
+    $contentDisposition = $response->headers->makeDisposition(
+        ResponseHeaderBag::DISPOSITION_ATTACHMENT,
+        'download.eml'
+    );
+    $response->headers->set('Content-Type', 'message/rfc822');
+    $response->headers->set('Content-Disposition', $contentDisposition);
+
+    return $response;
+
+.. note::
+
+    As it's possible for :class:`Symfony\\Component\\Mime\\DraftEmail`'s to be created
+    without a To/From they cannot be sent with the mailer.
+
 Development & Debugging
 -----------------------