linuxkpi.4, linuxkpi_wlan.4: add short man pages

Add brief documentation on LinuxKPI.
Add brief documentation on LinuxKPI 802.11 compat so we can refer to
something about which features are supported.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dumbbell, ziaee, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D50790