iavf(4): Improve man page
MFC after: 3 days
Reviewed by: erj
Differential Revision: https://reviews.freebsd.org/D43093
(cherry picked from commit ba222f6fb4b226ab24beaa13a8591dcb624ae70f)
(cherry picked from commit 4c380c44c82f36c04539e3c9e60e58567abf3856)
Approved by: re (karels)