wpa_supplicant.conf.5: add note about scan_ssid=1 eavesdropping

When scan_ssid=1 the list of configured SSIDs is available to
eavesdroppers. Note this in the man page.

PR: 194122
Reviewed by: debdrup, Pau Amma
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34576