Expand arp_responder help text
Expand arp_responder help text

Add additional text to arp_responder help to ensure that
proper documentation is included in the configuration guide.

Make ML2 and L2population references consistent.

Closes-Bug: #1321320

Change-Id: I0fc0691fc3729aa39888c90ca871ad7f711343f8

gkotton/vmware-nsx,gkotton/vmware-nsx