[omdb] fix wrong cooldown in omdb db instance show

[hawkw]

The omdb db instance show command currently displays the auto-restart cooldown duration as a time delta, calculated by the InstanceAutoRestart::can_reincarnate function. This function returns a TimeDelta with a comment suggesting that this is an absolute expiry timestamp, but it is actually the duration until that timestamp, calculated by subtracting the current Utc::now from the expiry time. This means that if Utc::now() in the omdb process ever returns a timestamp where now.signed_duration_since(last) is negative, the calculated cooldown duration will appear to be many years long. This branch fixes that by just making can_reincarnate return the absolute timestamp at which the cooldown will expire, which is what the doc comment on the enum says it is anyway. Since the timestamp value is only used by omdb anyway, this is a low-risk change.

I also added some additional help text to explain what these values mean.

Fixes #10660

[hawkw]

Huh...did I do that? I don't think I did...