Clarify torrc comment for IPv6Exit

## Uncomment this if you want your relay to be an exit, with the default

## exit policy (or whatever exit policy you set below).

## (If ReducedExitPolicy, ExitPolicy, or IPv6Exit are set, relays are exits.

## If neither exit policy option is set, relays are non-exits.)

## If none of these options are set, relays are non-exits.)

#ExitRelay 1

## Uncomment this if you want your relay to allow IPv6 exit traffic.