Update arp(4) to document the net.link.ether.inet.garp_rexmit_count sysctl.

Submitted by: David A. Bright <david.a.bright@dell.com>
Requested by: markj
Reviewed by: markj, badger
MFC after: 4 weeks
X-MFC with: r306577
Sponsored by: Dell EMC
Differential Revision: https://reviews.freebsd.org/D8136