document vzctl "--devnodes" option. Drop "--devices" example from documentation.

There is a functionality to manage access to /dev/ devices inside a Container via vzctl (corresponding prlctl functionality is also pending - OVZ-6518).

That way or another at the moment this functionality presents in vzctl, but its description is missing in docs (both in Users guide and command reference).

At the same time the option description presents in vzctl man page:
       Device access management

       --devnodes device:r|w|rw|none
           Give access (r - read, w - write, rw - read/write, none - no
           access) to special file /dev/device from CT.

Please add this description to documentation as well.

Another related issue:
"Users guide" has an example of using "--devices" vzctl option (in VPN configuration section). Need to substitute it with "--devnodes" option example.