Better batcher env var docs

[SebastienGllmt]

Right now we don't describe DEFAULT_VALIDATION_ACTIVE or GAME_NODE_URI

For GAME_NODE_URI, it's separate from BACKEND_URI so that you can use a specific node to just validate. Otherwise, as this operation is done pre-transaction, the node might be DOS'ed

[SebastienGllmt]

On a related note, STORAGE_CONTRACT_ADDRESS is a duplicate of CONTRACT_ADDRESS and I don't know of any reason why this was duplicated

EDIT: the reason is because the paima-engine env var name changed, and the batcher was never updated to reflect that. We should update the batcher as well