message: op-node: add better docstring for storage slot
  The unsafe block signer is stored at a magic storage
  slot that decouples `solc`'s storage layout system from
  being able to move the storage slot accidentally.
  This adds in a better docstring so that its obvious
  how the value is computed.
diff: @@ -15,7 +15,7 @@ import (
  var (
  // UnsafeBlockSignerAddressSystemConfigStorageSlot is the storage slot identifier of the unsafeBlockSigner
  - // `address` storage value in the SystemConfig L1 contract.
  + // `address` storage value in the SystemConfig L1 contract. Computed as `keccak256("systemconfig.unsafeblocksigner")`
  UnsafeBlockSignerAddressSystemConfigStorageSlot = common.HexToHash("0x65a7ed542fb37fe237fdfbdd70b31598523fe5b32879e307bae27a0bd9581c08")
  )
  