Update errors/tun.md to include instructions for device passthrough

[epic0421]

What is your suggestion?

As of Proxmox VE 8.2, a new option was added for device passthrough.

Old method of allowing access to /dev/net/tun in /etc/pve/lxc/x.conf

lxc.cgroup2.devices.allow: c 10:200 rwm
lxc.mount.entry: /dev/net dev/net none bind,create=dir
lxc.mount.entry: /dev/net/tun dev/net/tun none bind,create=file

New method of allowing access to /dev/net/tun in /etc/pve/lxc/x.conf

dev0: /dev/net/tun

Or it can be done through the gui in the lxc container

[epic0421]

Forgot to mention that this passthrough does work on unprivileged LXC containers too