Document behavior with `reneg-sec` setting for OpenVPN

[oxr463]

TL;DR - Every time the OpenVPN connection is disrupted, reneg-sec will re-auth, which requires 2FA (via this plugin).

Reference(s):

• https://forums.openvpn.net/viewtopic.php?t=21791

[oxr463]

This would be a good FAQ entry.