Cannot submit a voluntary exit for a future epoch

See https://discord.com/channels/593655374469660673/593655641445367808/1336809071797600346

> Does it have anything to do with the fact that I specified an --exitEpoch in the future? I'm looking at the Lodestar code and see this line (https://github.com/ChainSafe/lodestar/blob/9c66fac4a47446b225c874997dfc4d7b93a820b3/packages/state-transition/src/block/processVoluntaryExit.ts#L43C5-L44C43) in the logic that would result in the VOLUNTARY_EXIT_ERROR_INVALID:
> ```
> // exits must specify an epoch when they become valid; they are not valid before then
> currentEpoch >= voluntaryExit.epoch &&
> ```
> But how else would you specify a future-dated exit? Is that actually not supported, and --exitEpoch is a flag that can't be used in practice?

> Might recommend removing that flag from the documentation if in practice it either does nothing (past-dated epoch) or breaks the command (future dated epoch)