dm: add a note about DM to sql_require_primary_key (pingcap#15993)

dm/task-configuration-file-full.md

@@ -45,6 +45,7 @@ target-database:                # Configuration of the downstream database insta
     sql_mode: "ANSI_QUOTES,NO_ZERO_IN_DATE,NO_ZERO_DATE" # Since DM v2.0.0, if this item does not appear in the configuration file, DM automatically fetches a proper value for "sql_mode" from the downstream TiDB. Manual configuration of this item has a higher priority.
     tidb_skip_utf8_check: 1                     # Since DM v2.0.0, if this item does not appear in the configuration file, DM automatically fetches a proper value for "tidb_skip_utf8_check" from the downstream TiDB. Manual configuration of this item has a higher priority.
     tidb_constraint_check_in_place: 0
+    sql_require_primary_key: OFF                # Controls whether a table must have a primary key at the session level. During the creation of a DM task, several metadata tables are created in TiDB, and some of them have no primary key. If this parameter is enabled, these metadata tables without primary keys cannot be created, which will cause DM to fail to create the task. In this case, you need to set this parameter to `OFF`.
   security:                       # The TLS configuration of the downstream TiDB
     ssl-ca: "/path/to/ca.pem"
     ssl-cert: "/path/to/cert.pem"

system-variables.md

@@ -922,6 +922,12 @@ mysql> SHOW GLOBAL VARIABLES LIKE 'max_prepared_stmt_count';
 - This feature is based on the similarly named [`sql_require_primary_key`](https://dev.mysql.com/doc/refman/8.0/en/server-system-variables.html#sysvar_sql_require_primary_key) in MySQL 8.0.
 - It is strongly recommended to enable this variable when using TiCDC. This is because replicating changes to a MySQL sink requires that tables have a primary key.
 
+<CustomContent platform="tidb">
+
+- If you enable this variable and are using TiDB Data Migration (DM) to migrate data, it is recommended that you add `sql_require_ primary_key` to the `session` part in the [DM Task Configuration File](/dm/task-configuration-file-full.md#task-configuration-file-template-advanced) and set it to `OFF`. Otherwise, it will cause DM to fail to create tasks.
+
+</CustomContent>
+
 ### sql_select_limit <span class="version-mark">New in v4.0.2</span>
 
 - Scope: SESSION | GLOBAL