Update the multi-schema change DDL limitation

mysql-compatibility.md

@@ -104,7 +104,7 @@ See also: [TiDB SQL Grammar](https://pingcap.github.io/sqlgram/#functioncallkeyw
 
 In TiDB, all supported DDL changes are performed online. Compared with DDL operations in MySQL, the DDL operations in TiDB have the following major restrictions:
 
-* Multiple operations cannot be completed in a single `ALTER TABLE` statement. For example, it is not possible to add multiple columns or indexes in a single statement. Otherwise, the `Unsupported multi schema change` error might be output.
+* It is not supported to alter the same column or index when altering multiple schemas in a single `ALTER TABLE` statement. For example, when you run the `ALTER TABLE t1 MODIFY COLUMN c1 INT, DROP COLUMN c1` command, a `Unsupported operate same column/index` error is output.
 * `ALTER TABLE` in TiDB does not support the changes of some data types. For example, TiDB does not support the change from the `DECIMAL` type to the `DATE` type. If a data type change is unsupported, TiDB reports the `Unsupported modify column: type %d not match origin %d` error. Refer to [`ALTER TABLE`](/sql-statements/sql-statement-modify-column.md) for more details.
 * The `ALGORITHM={INSTANT,INPLACE,COPY}` syntax functions only as an assertion in TiDB, and does not modify the `ALTER` algorithm. See [`ALTER TABLE`](/sql-statements/sql-statement-alter-table.md) for further details.
 * Adding/Dropping the primary key of the `CLUSTERED` type is unsupported. For more details about the primary key of the `CLUSTERED` type, refer to [clustered index](/clustered-indexes.md).

sql-statements/sql-statement-add-column.md

@@ -86,7 +86,6 @@ mysql> SELECT * FROM t1;
 
 ## MySQL compatibility
 
-* Adding multiple columns at the same time in a statement is currently not supported.
 * Adding a new column and setting it to the `PRIMARY KEY` is not supported.
 * Adding a new column and setting it to `AUTO_INCREMENT` is not supported.
 * There are limitations on adding generated columns, refer to: [generated column limitations](/generated-columns.md#limitations).

sql-statements/sql-statement-add-index.md

@@ -77,7 +77,6 @@ mysql> EXPLAIN SELECT * FROM t1 WHERE c1 = 3;
 
 * `FULLTEXT`, `HASH` and `SPATIAL` indexes are not supported.
 * Descending indexes are not supported (similar to MySQL 5.7).
-* Adding multiple indexes at the same time is currently not supported.
 * Adding the primary key of the `CLUSTERED` type to a table is not supported. For more details about the primary key of the `CLUSTERED` type, refer to [clustered index](/clustered-indexes.md).
 
 ## See also

sql-statements/sql-statement-alter-table.md

@@ -157,7 +157,7 @@ Query OK, 0 rows affected, 1 warning (0.25 sec)
 
 The following major restrictions apply to `ALTER TABLE` in TiDB:
 
-- Making multiple changes in a single `ALTER TABLE` statement is currently not supported.
+- Changes of the same column or index when altering multiple schemas in a single statement are not supported.
 
 - Changes of the [Reorg-Data](/sql-statements/sql-statement-modify-column.md#reorg-data-change) types on primary key columns are not supported.
 

sql-statements/sql-statement-change-column.md

@@ -149,7 +149,6 @@ ERROR 8200 (HY000): Unsupported modify column: change from original type decimal
 
 ## MySQL compatibility
 
-* Making multiple changes in a single `ALTER TABLE` statement is currently not supported.
 * Changes of [Reorg-Data](/sql-statements/sql-statement-modify-column.md#reorg-data-change) types on primary key columns are not supported.
 * Changes of column types on partitioned tables are not supported.
 * Changes of column types on generated columns are not supported.

sql-statements/sql-statement-drop-column.md

@@ -75,7 +75,6 @@ mysql> SELECT * FROM t1;
 
 ## MySQL compatibility
 
-* Dropping multiple columns in the same statement is not supported.
 * Dropping primary key columns or columns covered by the composite index is not supported.
 
 ## See also

sql-statements/sql-statement-modify-column.md

@@ -169,13 +169,6 @@ CREATE TABLE `t1` (
 
 ## MySQL compatibility
 
-* Does not support modifying multiple columns using a single `ALTER TABLE` statement. For example:
-
-    ```sql
-    ALTER TABLE t1 MODIFY col1 BIGINT, MODIFY id BIGINT NOT NULL;
-    ERROR 1105 (HY000): Unsupported multi schema change
-    ```
-
 * Does not support modifying the Reorg-Data types on the primary key columns but supports modifying the Meta-Only types. For example:
 
     ```sql

system-variables.md

@@ -826,22 +826,6 @@ Constraint checking is always performed in place for pessimistic transactions (d
 - Default value: `OFF`
 - This variable is used to determine whether to include the `AUTO_INCREMENT` columns when creating a generated column or an expression index.
 
-### tidb_enable_change_multi_schema
-
-> **Warning:**
->
-> TiDB will support more types of multi-schema changes in the future. This system variable will be deleted in a future release of TiDB.
-
-- Scope: GLOBAL
-- Persists to cluster: Yes
-- Type: Boolean
-- Default value: `OFF`
-- This variable is used to control whether multiple columns or indexes can be altered in one `ALTER TABLE` statement. When the value of this variable is `ON`, only the following multi-schema changes are supported:
-    - Add multiple columns. For example, `ATLER TABLE t ADD COLUMN c1 INT, ADD COLUMN c2 INT;`.
-    - Drop multiple columns. For example, `ATLER TABLE t DROP COLUMN c1, DROP COLUMN c2;`.
-    - Drop multiple indexes. For example, `ATLER TABLE t DROP INDEX i1, DROP INDEX i2;`.
-    - Drop a column covered by a single-column index. For example, `ALTER TABLE t DROP COLUMN c1`, in which the schema contains `INDEX idx(c1)`.
-
 ### tidb_enable_cascades_planner
 
 > **Warning:**