doc: add transfer rules VXLAN actions in sfc guide

doc: add transfer rules VXLAN actions in sfc guide

Actions VXLAN_DECAP and VXLAN_ENCAP need to be listed
among actions supported for transfer flows.

Fixes: 6ab6c40d1e83 ("net/sfc: support action VXLAN decap in transfer rules")
Fixes: 1bbd1ec2348a ("net/sfc: support action VXLAN encap in MAE backend")
Cc: stable@dpdk.org
Signed-off-by: Ivan Malov <ivan.malov@oktetlabs.ru>
Reviewed-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: Andy Moreton <amoreton@xilinx.com>