add note about total balance overflowing

specs/phase0/beacon-chain.md

@@ -954,6 +954,7 @@ def get_beacon_proposer_index(state: BeaconState) -> ValidatorIndex:
 def get_total_balance(state: BeaconState, indices: Set[ValidatorIndex]) -> Gwei:
     """
     Return the combined effective balance of the ``indices``. (1 Gwei minimum to avoid divisions by zero.)
+    Math safe up to ~10B ETH, afterwhich this overflows uint64.
     """
     return Gwei(max(1, sum([state.validators[index].effective_balance for index in indices])))
 ```