mergemaster: add deprecation notice to man page

PR: 252417
Reviewed by: bapt, gjb, kevans (all earlier)
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D23327

(cherry picked from commit 398b12691b4f1e7a73afca4c051c182bfcaf9671)
(cherry picked from commit ab53efd14237b71c3c0e4e212ada6526ef14402c)