iwn.4: move hardware to HARDWARE + minor cleanup

Add devices supported by this driver to a HARDWARE section
for inclusion in the release Hardware Compatibility Notes.
While here:

• add SPDX tag
• add networking(7) cross-ref quick start guide
• tweak examples for consistency

Differential Revision: https://reviews.freebsd.org/D47168

(cherry picked from commit 545dbf9d5fadfab591350c7c504b8e4bd113ba7b)