tidb_row_checksum(): Update docs

functions-and-operators/tidb-functions.md

@@ -333,12 +333,14 @@ To enable the checksum feature of single-row data in TiDB (controlled by the sys
 SET GLOBAL tidb_enable_row_level_checksum = ON;
 ```
 
+This configuration only takes effect for newly created sessions, so you need to reconnect to TiDB.
+
 Create table `t` and insert data:
 
 ```sql
 USE test;
-CREATE TABLE t (id INT PRIMARY KEY, k INT, c int);
-INSERT INTO TABLE t values (1, 10, a);
+CREATE TABLE t (id INT PRIMARY KEY, k INT, c CHAR(1));
+INSERT INTO t values (1, 10, 'a');
 ```
 
 The following statement shows how to query the checksum value of the row where `id = 1` in table `t`:

system-variables.md

@@ -2771,6 +2771,8 @@ Query OK, 0 rows affected (0.09 sec)
 
 </CustomContent>
 
+- You can use the [`TIDB_ROW_CHECKSUM()`](/functions-and-operators/tidb-functions.md#tidb_row_checksum) function to get the checksum value of a row.
+
 ### tidb_enforce_mpp <span class="version-mark">New in v5.1</span>
 
 - Scope: SESSION