iwm.4: move hardware list to .Sh HARDWARE and minor cleanup

Add devices supported by this driver to a HARDWARE section
for inclusion in the Release Hardware Compatibility Notes.

While here:

• license: add spdx license identifier tag
• synopsis: join a line for grep/brevity
• description: reflow for consistency, xref iwmfw(4)
• examples: use test-net-1 in cidr notation as example IPv4 address
• see also: add networking(7) quick start guide
• bugs: move 'incompatible modes' here for consistency/flow
• mention 802.11a support

MFC after: 3 days
Reported by: bz (relnotes generation, cidr, test-net-1 ip4addr)
Reported by: Graham Percival <gperciva@tarsnap.com> (no prompt)
Reviewed by: bz
Differential Revision: https://reviews.freebsd.org/D47171