docs: update cnat docs to current vpp version
docs: update cnat docs to current vpp version

The documentation for the cnat plugin is highly outdated specially on
the snat section.

Type: docs

Signed-off-by: Miguel Borges de Freitas <027cccb703202b9f2ca73da0bbb5008ba89b1c7a@alticelabs.com>
Change-Id: I30b0c6295d3c812b636374753af3c37f29b0cc53

FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,chrisy/vpp,FDio/vpp,FDio/vpp,chrisy/vpp,FDio/vpp