vnet: update help message for intfc state
vnet: update help message for intfc state

Add useful help information on set interface state.

Change-Id: Ibcdcea2849ccaee4cc72bf38d79102beb0d4be11
Signed-off-by: Ray Kinsella <062716053169c18af5393a3d798a58342ae656ed@intel.com>

chrisy/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,vpp-dev/vpp,vpp-dev/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,vpp-dev/vpp,vpp-dev/vpp