Document tunable net.link.epair.ether_gen_addr

Approved by: hrs
Fixes: 590493c1419092e98f7ad1dcadb886973502341e ("if_epair(4): use ether_gen_addr(9) for stable MAC address")
MFC after: 1 week (together with the tunable)
Differential Revision: https://reviews.freebsd.org/D51861