Add comment explaining the `1_000_000` constant

op-chain-ops/crossdomain/migrate.go

@@ -135,6 +135,10 @@ func MigrateWithdrawalGasLimit(data []byte, chainID *big.Int) uint64 {
 			// Constant overhead
 			RelayConstantOverhead +
 				// Dynamic overhead (EIP-150)
+				// We use a constant 1 million gas limit due to the overhead of simulating all migrated withdrawal
+				// transactions during the migration. This is a conservative estimate, and if a withdrawal
+				// uses more than the minimum gas limit, it will fail and need to be replayed with a higher
+				// gas limit.
 				(MinGasDynamicOverheadNumerator*1_000_000)/MinGasDynamicOverheadDenominator +
 				// Gas reserved for the worst-case cost of 3/5 of the `CALL` opcode's dynamic gas
 				// factors. (Conservative)