man: document ether_gen_addr(9)

This KPI is used to assign a MAC address to an interface that doesn't
already have one assigned.

Reviewed by: bcr, gnn, imp, kbowling, kp
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29787

(cherry picked from commit e58a65ccdac352712e19a60fffa57a86afabbde9)