ipv6 issue - documentation update

[shuther]

because I disabled ipv6 at the router level I had to apply one change:

• recent version of tailscale enforces a change in /etc/rc.conf - set manually: ipv6_activate_all_interfaces="NO" (without it I am losing the internet connection)
  It could make sense to set the mac address when we create the jail so it can be added at the DHCP server side.
  The expiration of the key (for the authentification: one day?) and for the machines (no limit) could be added?