[Mailer] Document the new S/MIME encrypter options and the listener priorities

[Spomky]

Documents symfony/symfony#64541, which aligned the S/MIME listeners with the PGP/MIME design.

In mailer.rst, the "Encrypting Messages Globally" section is completed with the options added in 8.2:

• certificates, to list the recipient certificates directly in the configuration instead of writing a repository service;
• encrypt_for_sender, to also encrypt for the sender so it can read the messages it sent;
• on_missing_certificate and its per message override through the X-SMime-Encrypt header, in a new "Recipients Without a Certificate" section, with the deprecation of the send_unencrypted default and the caveat that framework.mailer.envelope.recipients is applied after the skip behavior.

A new "Signing and Encrypting the Same Message" section documents the PRIORITY constants of SmimeSignedMessageListener, SmimeEncryptedMessageListener and DkimSignedMessageListener, and the fact that DKIM moved from -128 to -228 so it signs the encrypted message.

The section also said that the global S/MIME encrypter "automatically applies to all outgoing messages", which was never true: the listener only encrypts the messages carrying the X-SMime-Encrypt header. That header was not documented at all, so it is documented here rather than in a separate PR.

reference/configuration/framework.rst gets the same three options in the smime_encrypter reference.

PGP/MIME is documented separately in #22732, so this PR leaves it out.

Fixes #22719