iwm(4): Document limitations of the driver

Document that iwm(4) currently doesn't support 802.11n and 802.11ac.

PR: 247874
Submitted by: Charles Ross <cwr at sdf dot org>
Reviewed by: brueffer, markj
Approved by: brueffer
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25666