ixl.4 + ice.4: intro sentence for HARDWARE notes

MFC after: 3 days
Pull Request: https://github.com/freebsd/freebsd-src/pull/1495

Add man page for the ice network driver.

PR: 262892
MFC after: 3 days
Reviewed by: concussious.bugzilla@runbox.com, erj
Differential Revision: https://reviews.freebsd.org/D45270