sql: add 8003 error code description (#588)

Via: pingcap/docs-cn#839

sql/error.md

@@ -16,6 +16,7 @@ TiDB is compatible with the error codes in MySQL, and in most cases returns the
 | ---- | ------- | --------- |
 | 8001 | The memory used by the request exceeds the threshold limit for the TiDB memory usage. | Increase the value of the system variable with the `tidb_mem_quota` prefix. |
 | 8002 | To guarantee consistency, a transaction with the `SELECT FOR UPDATE` statement cannot be retried when it encounters a commit conflict. TiDB rolls back the transaction and returns this error. | Retry the failed transaction. |
+| 8003 | If the data in a row is not consistent with the index when executing the `ADMIN CHECK TABLE` command, TiDB returns this error. |
 | 9001 | The PD request timed out. | Check the state/monitor/log of the PD server and the network between the TiDB server and the PD server. |
 | 9002 | The TiKV request timed out. | Check the state/monitor/log of the TiKV server and the network between the TiDB server and the TiKV server. |
 | 9003 | The TiKV server is busy and this usually occurs when the workload is too high. | Check the state/monitor/log of the TiKV server. |