wifi manuals: Mlink + document description consistency

Interfaces all have an mlink to if_$foo. Add these for the missing ones
and remove an incorrect one from rtwn_pci. Wireless network drivers are
all accessible via `apropos -s4 "wireless network driver", except two
which are "wireless network device". I actually prefer the latter, but
make them all consistent upon the more common parlance. Tag SPDX on one
of the files I touched, while here.

MFC after: 3 days
Reviewed by: bz, carlavilla, mhorne
Approved by: carlavilla, mhorne (mentors)
Differential Revision: https://reviews.freebsd.org/D49063

(cherry picked from commit 4262dbc57982383eb61a8b7806de6dd4b7802da8)