Make relays into exits when ExitRelay is auto and IPv6Exit is 1
In #21530 (moved), we made relays exit if ExitRelay is auto, and either:
- ReducedExitPolicy is 1, or
- ExitPolicy is set
But a relay operator who sets IPv6Exit to 1 also wants to be an exit.
So we should add
IPv6Exit 1 to the options that activate exits when ExitRelay is auto. We should also update the documentation (see #29612 (moved)).