isp(4): Document support for new devices

Add 28xx based HBAs to manpage.

PR: 271062
Reviewed by: imp, mav
Sponsored by: Technical University of Munich
Pull Request: https://github.com/freebsd/freebsd-src/pull/726

(cherry picked from commit 2b4ccfbbc65b2f140fab6db998e9654c5c2bb972)