Add description how the filter works in case of multiple interfaces with the same MAC.
Add description how the filter works in case of multiple interfaces
with the same MAC.

PR:     kern/109815
MFC after:	7 days
Submitted by:	Eygene Ryabinkin
Glanced by:	rik (all pointy hats are mine)
Discussed with:	julian@, rik@

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase