sysctl.conf(5): Mention sysctl.conf.local in the sysctl.conf(5) manual page

sysctl.conf(5): Mention sysctl.conf.local in the sysctl.conf(5) manual page

The possibility of using a sysctl.conf.local on a machine that has a shared
sysctl.conf(5) isn't documented. So mention the sysctl.conf.local in the
manual page.

PR:	254901
Submitted by:	Jose Luis Duran <jlduran at gmail dot com>
Reported by:	Jose Luis Duran <jlduran at gmail dot com>
Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D29673

(cherry picked from commit 46c99528078ad478f50910110a933abef0b89cde)