[Mailer] Update mailer.rst

[GrinWay]

This piece of addition information will solve the problem of misunderstanding: Why my emails can't be processed asynchronously.

[smnandre]

I fear this addition would add some confusion.

In fact, the very next line of the documentation states the opposite:

Note

If your code used Symfony\Component\Mailer\MailerInterface, you need to replace it by Symfony\Component\Mailer\Transport\TransportInterface to have the SentMessage object returned.

So maybe find a better place?

[GrinWay]

Fixed