packages: Add kernel-man to minimal-jail

These manpages are broadly applicable to all systems, even jails,
since they describe kernel facilities which are available in jails.

MFC after: 3 days
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D52743

(cherry picked from commit 29a4fe3dbd8493d0066c5424bfe0e890d4703f34)