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:	freebsd/freebsd-src#1368

share/man/man5/rc.conf.5

@@ -618,6 +618,15 @@ When specified, this variable overrides the
 variable for interface
 .Ar iface
 only.
+.It Va dhclient_arpwait
+.Pq Vt bool
+Set to
+.Dq Li NO
+to stop
+.Xr dhclient 8
+from waiting for ARP resolution, to make the system boot faster.
+This may be done on networks where the DHCP server is certain to
+know whether an address is available.
 .It Va synchronous_dhclient
 .Pq Vt bool
 Set to