[Mailer] Fixed and clarified custom Content-ID feature documentation

Author: dakujem

mailer.rst

@@ -658,16 +658,18 @@ images inside the HTML contents::
         ->html('... <div background="cid:footer-signature"> ... </div> ...')
     ;
 
+The actual Content-ID value present in the e-mail source will be randomly generated by Symfony.
+
 You can also use the :method:`DataPart::setContentId() <Symfony\\Component\\Mime\\Part\\DataPart::setContentId>`
 method to define a custom Content-ID for the image and use it as its ``cid`` reference::
 
     $part = new DataPart(new File('/path/to/images/signature.gif'));
-    $part->setContentId('footer-signature');
+    $part->setContentId('footer-signature@my-app');
 
     $email = (new Email())
         // ...
         ->addPart($part->asInline())
-        ->html('... <img src="cid:footer-signature"> ...')
+        ->html('... <img src="cid:footer-signature@my-app"> ...')
     ;
 
 .. versionadded:: 6.1