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

(cherry picked from commit 8e5e42d54ad3093e38263814d1f3e7d8406d5421)
(cherry picked from commit 42e50eaf7f6f4459f1217564fd143ae4f8127154)

Approved by: re (karels)