linuxkpi.4, linuxkpi_wlan.4: add short man pages and connect to build

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
Reviewed by: dumbbell, ziaee, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D50790

(cherry picked from commit bfaec3e6474139c592f39b4adbdbeb35c9026c25)
(cherry picked from commit 1563b306d19db25f5bbf071897b60429b192040c)
(cherry picked from commit 8b399d613f5420bb7e57be94806e156e4c1579fb)