cdce: add to cdce.4 information about NCM network mode

add a new section how to use NCM network mode and specify the required
kernel modules

Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/646
Approved by: re (cperciva)

(cherry picked from commit 58e1d01022415e29d3e08c4d3936b6631c27406c)
(cherry picked from commit 6c9ad7b8bc7dbc17f41ddab6779548fb2728608a)