Partial fix for #1701

Clarify that state transitions with `uint64` overflows are invalid.

specs/phase0/beacon-chain.md

@@ -1195,7 +1195,7 @@ Let `genesis_block = BeaconBlock(state_root=hash_tree_root(genesis_state))`.
 
 ## Beacon chain state transition function
 
-The post-state corresponding to a pre-state `state` and a signed block `signed_block` is defined as `state_transition(state, signed_block)`. State transitions that trigger an unhandled exception (e.g. a failed `assert` or an out-of-range list access) are considered invalid.
+The post-state corresponding to a pre-state `state` and a signed block `signed_block` is defined as `state_transition(state, signed_block)`. State transitions that trigger an unhandled exception (e.g. a failed `assert` or an out-of-range list access) are considered invalid. State transitions that cause a `uint64` overflow are also considered invalid.
 
 ```python
 def state_transition(state: BeaconState, signed_block: SignedBeaconBlock, validate_result: bool=True) -> BeaconState: