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
MFC after: 3 days
Reviewed by: ziaee, emaste (comments)
Differential Revision: https://reviews.freebsd.org/D50319