Document optional Dovecot mail deletion logging - #1111

Summary

• include optional mail_log_events and mail_log_fields settings in the Dovecot 2.4 example
• keep detailed logging and the required mail_log/notify plugins commented out by default
• explain how administrators can explicitly enable the complete set
• link to the official Dovecot documentation
• document the prospective nature and privacy implications of sender and subject fields

Why

Detailed deletion logging can help investigate future reports of missing mail,
but sender and subject metadata may be sensitive. The example documents the
capability without enabling it by default.

Validation

• git diff --check
• verified all related settings remain commented out in the example
• verified the events and fields against the official Dovecot 2.4.1 documentation

doveconf -n was not run because Dovecot is unavailable on the Windows host.