dhclient: Update rc.conf.5 with dhclient_arpwait

Add new dhclient_arpwait option to rc.conf.5, with information about
what it does, and cases in which it could be disabled.

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