arp(8) clarify the use of the '-a flag'

PR: 238146
Submitted by: Andrew Stevenson <andrew at ugh dot net dot au>
Reported by: Andrew Stevenson <andrew at ugh dot net dot au>
Reviewed by: bcr (mentor)
Approved by: bcr (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25645