minor #19946 [Mailer] Fixed and clarified custom Content-ID feature documentation (dakujem)

This PR was merged into the 6.4 branch.

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

> Addresses [symfony/symfony #54162](symfony/symfony#54162)

The documentation is incorrect, as it shows calls to `DataPart::setContentId` with strings that are rejected by [the method's implementation](https://github.com/symfony/symfony/blob/7.2/src/Symfony/Component/Mime/Part/DataPart.php#L66), because they do not contain a required `@` character.

![image](https://github.com/symfony/symfony/assets/443067/560fd816-e4d2-4a48-8d82-ef7dfa38db16)

I also added a sentence clarifying that the actual Content-ID value will be ramdom-generated by Symfony when not using a custom Content-ID.

This feature has been added in `v6.3` (according to the documentation) and the documentation is incorrect throughout versions `6.3`, `6.4`, `7.0`, `7.1` and `7.2`.
I'm not sure how to distribute this PR to all other branches - should I create separate PRs for each?

Commits
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468a9ac [Mailer] Fixed and clarified custom Content-ID feature documentation

Author: javiereguiluz

mailer.rst

@@ -682,16 +682,18 @@ 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