[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

[javiereguiluz]

@GrinWay thanks ... and congrats on your first Symfony Docs contribution. Sorry it took us so long to merge it.

While reviewing this, we realized that we had to reword the entire section to explain things more clearly. So, we did that while merging. See https://github.com/symfony/symfony-docs/commit/74a146ee4f74fbc7dae98fa695040dd8da81dcc3

Thanks!

[GrinWay]

F - Fantastic