[Mailer] Document `In-Reply-To` and `References` headers

`In-Reply-To` and `References` headers aren't explained at all at https://symfony.com/doc/4.4/mailer.html

Especially adding *multiple* values isn't self-explanatory. In order to get this:
```
References: <1234@local.machine.example> <3456@example.net>
```
... you need to do:

```php
$headers->addIdHeader('References', ['1234@local.machine.example', '3456@example.net']);
```
