Add gone_in(9) man page

Reviewed by: imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30703

(cherry picked from commit d8aeab4b45a94b3e1ab4396235e4426747e310fe)
(cherry picked from commit 1bacdee058ae2767e9fba5ae7593ce1518554391)