[Mailer] Minor tweaks

mailer.rst

@@ -667,8 +667,8 @@ file or stream::
 Use the ``asInline()`` method to embed the content instead of attaching it.
 
 The second optional argument of both methods is the image name ("Content-ID" in
-the MIME standard). Its value is an arbitrary string used later to reference the
-images inside the HTML contents::
+the MIME standard). Its value is an arbitrary string that must be unique in each
+email message and is used later to reference the images inside the HTML contents::
 
     $email = (new Email())
         // ...
@@ -683,11 +683,11 @@ images inside the HTML contents::
     ;
 
 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'));
+    // according to the spec, the Content-ID value must include at least one '@' character
     $part->setContentId('footer-signature@my-app');
 
     $email = (new Email())