man4: Add a manual page for kvmclock

Reviewed by: pauamma, imp, kib
MFC after: 2 weeks
Sponsored by: Klara, Inc.
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D38343

(cherry picked from commit a2286a1f4651f9369cacb82bb1777920f5a86fdc)