cmd/global: Add help messages for L3VPN and MPLS
cmd/global: Add help messages for L3VPN and MPLS

Signed-off-by: Satoshi Fujimoto <cd75558e8cccb245feb2368a5a53f09e4ecc2257@gmail.com>

tamihiro/gobgp,j-kato/gobgp,tamihiro/gobgp,a16/gobgp,tamihiro/gobgp,osrg/gobgp,a16/gobgp,fujita/gobgp,osrg/gobgp,tamihiro/gobgp,j-kato/gobgp,a16/gobgp,fujita/gobgp,fujita/gobgp,a16/gobgp,osrg/gobgp,a16/gobgp,a16/gobgp,osrg/gobgp,fujita/gobgp,a16/gobgp,j-kato/gobgp