[Mailer] Document the S/MIME listeners alignment with PGP/MIME

[lacatoire]

Documents the changes made in symfony/symfony#64541.

• the new certificates map, on_missing_certificate (with the deprecation of its send_unencrypted default) and encrypt_for_sender options of framework.mailer.smime_encrypter, and the per-message X-SMime-Encrypt override;
• the deterministic ordering of the listeners through their new PRIORITY constants: S/MIME signing (-128), then S/MIME encryption (-200), then DKIM (-228) on the transmitted message;
• the matching entries in the framework configuration reference.

Note: the fixes to the pre-existing part of that section (the X-SMime-Encrypt header requirement and the missing enabled: true) are sent separately against 7.4 in #22737, as they apply to the maintained branches too. This PR uses the same wording, so merging up should be conflict-free.

Fixes #22719