[lisp] To help converge on the UDP checksum text proposed for -04

[Dino Farinacci <dino at cisco.com>]

Let's start with this text. Please wordsmith it:

   UDP Checksum:  this field MAY be transmitted as zero by an ITR and
      when received by an ETR, MUST accept the packet for  
decapsulation.
      When an ITR transmits a non-zero value, an ETR MAY verify the
      checksum value.  If the checksum is verified, the packet is
      accepted for decapsulation, otherwise, it is silently dropped.
      Note, even when the UDP checksum is transmitted as zero an
      intervening NAT device can recalculate the checksum and rewrite
      the UDP checksum field to non-zero.  See draft [UDP-TUNNELS] for
      details.

Thanks,
Dino