reference/configuration: add tidb_row_format_version (#2290)

reference/configuration/tidb-server/tidb-specific-variables.md

@@ -351,6 +351,14 @@ Usage of statements:
 - `CREATE TABLE`: `CREATE TABLE t (c int) SHARD_ROW_ID_BITS = 4;`
 - `ALTER TABLE`: `ALTER TABLE t SHARD_ROW_ID_BITS = 4;`
 
+### tidb_row_format_version
+
+- Scope: GLOBAL
+- Default value: `2`
+- Controls the format version of the newly saved data in the table. In TiDB v4.0, the [new storage row format](https://github.com/pingcap/tidb/blob/master/docs/design/2018-07-19-row-format.md) version `2` is used by default to save new data.
+- If you upgrade from a TiDB version earlier than 4.0.0 to 4.0.0, the format version is not changed, and TiDB continues to use the old format of version `1` to write data to the table, which means that **only newly created clusters use the new data format by default**.
+- Note that modifying this variable does not affect the old data that has been saved, but applies the corresponding version format only to the newly written data after modifying this variable.
+
 ## tidb_slow_log_threshold
 
 - Scope: SESSION