message: specs: update to include `WithdrwalInitiatedExtension1`
diff: @@ -87,6 +87,8 @@ interface L2ToL1MessagePasser {
  bytes data
  );
  + event WithdrawalInitiatedExtension1(bytes32 indexed hash);
  +
  event WithdrawerBalanceBurnt(uint256 indexed amount);
  function burn() external;
  @@ -100,6 +102,14 @@ interface L2ToL1MessagePasser {
  ```
  +The `WithdrawalInitiated` event includes all of the data that is hashed and
  +stored in the `sentMessages` mapping. The `WithdrawalInitiatedExtension1` emits
  +the hash that was computed and used as part of the storage proof used to
  +finalize the withdrawal on L1.
  +
  +The events are separate as to preserve backwards compatibility. The hashing
  +scheme could be upgraded in the future through a contract upgrade.
  +
  ### Addresses are not Aliased on Withdrawals
  [address-aliasing]: #no-address-aliasing
  