doc: Mention that veth.pair is ignored for unpriv

veth.pair is ignore for unprivileged containers as allowing an
unprivileged user to set a specific device name would allow them to
trigger actions in tools like NetworkManager or other uevent based
handlers that may react based on specific names or prefixes being used.

Signed-off-by: Stéphane Graber <stgraber@ubuntu.com>