wlanstats: update usage, improve man page

wlanstats prints the wrong usage statement, including a nonexistent -a
flag (*) and a description of -m that suggests multiple arguments.
Make the manpage slightly more clear.

(*) the implementation of that is #if 0 and the option was disabled

in 530c13c5401c.

Sponsored by: The FreeBSD Foundation (commit)
PR: 285413
MFC after: 3 days
Reviewed by: ziaee
Differential Revision: https://reviews.freebsd.org/D49365