wlanstats: add man page

After two decades in tools add a basic man page to wlanstats now that
it is in usr.sbin. Most users will likely ever only run it without
arguments.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
X-MFC with: 61ce422531805
Reviewed by: adrian, ziaee
Differential Revision: https://reviews.freebsd.org/D49186