iwlwifi: clarify page update

Based on some feedback clarify the man page for

• how to load the driver currently
• status of the driver with respect to iwm(4)

and leave a comment to (automatically) add a full list of chipsets
to the man page.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: debdrup
Differential Revision: https://reviews.freebsd.org/D33713