iwlwifi.4: Update modes for recent 802.11ac

By: ziaee
Reported by: Pat Maddox <pat@patmaddox.com>
Reviewed by: bz (previous version), emaste, mhorne
Differential Revision: https://reviews.freebsd.org/D49937

iwlwifi: update man pages for generations, firmware. and 11n/11ac

Put Intel's iwlwifi better in context to iwm, iwx and iwn.
Document the important tunables so users know where to go.
Document the port/package for firmware where firmware is now
maintained.
Adjust wording and table for the firmware and a flavor column.
Update History sections for 11n/11ac support as well as firmware
module removal from base.

Sponsored by: The FreeBSD Foundation
Reviewed by: ziaee, emaste (comments)
Differential Revision: https://reviews.freebsd.org/D50319

Approved by: re (cperciva)

(cherry picked from commit c33945a112c84795c7bd87a8e94d0cf322ef5000)
(cherry picked from commit a7e183c0c9a5579cdaa3e2ff9597e5e3d41dbdb4)
(cherry picked from commit 4ae64bd33f24cff90ff75efe67176a6df1c3af62)