Make relays into exits when ExitRelay is auto and IPv6Exit is 1

In legacy/trac#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 legacy/trac#29612 (moved)).

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