dhclient: Update dhclient man page for n flag

Document new n flag for disabling ARP resolution within dhclient.

Sponsored by: Google LLC (GSoC 2024)
Signed-off-by: Isaac Cilia Attard <icattard@FreeBSD.org>
MFC after: 10 days
Reviwed by: cperciva, brooks, Tom Hukins, Alexander Ziaee
Pull Request: https://github.com/freebsd/freebsd-src/pull/1368

(cherry picked from commit 84fce4b67adc1ed1882d9371eb5d92dbbc79766d)