[Mailer] mention that BCC recipients do not work with DKIM signing

[kickofitall]

I think it should be mentioned that BCC receivers don't work with DKIM signing.
A caution block would be appropriate.
Source: symfony/symfony#44006 (comment)

[sdespont]

OK, thanks for adding this precision in the doc. But I don't understand why it was possible to send BCC for signed messages with SwiftMailer ? Was it a mistake ?