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.

op-node/node/runtime_config.go

@@ -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")
 )