iwlwifi.4: add a note on how to bootstrap firmware files

In case someone ends up without any networking and no firmware files
add short instructions on how firmware can be bootstrapped manually.
This was prompted by PR287393 and D50777 during the 14.3-RELEASE days
after firmware was removed from the stable/14 and only available via
ports/package/fwget.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: ziaee, pauamma_gundo.com
Differential Revision: https://reviews.freebsd.org/D50791