sql: update tidb_batch_insert/delete autocommit (#462)

* sql: update tidb_batch_insert autocommit

Via: pingcap/docs-cn#721

* sql: update tidb_batch_insert autocommit

Author: lilin90

sql/tidb-specific.md

@@ -128,15 +128,15 @@ If you need to set the global variable, run:
 
 - Scope: SESSION | GLOBAL
 - Default value: 0
-- This variable is used to set whether to divide the inserted data automatically.
+- This variable is used to set whether to divide the inserted data automatically. It is valid only when `autocommit` is enabled.
 - When inserting a large amount of data, you can set the variable value to true. Then the inserted data is automatically divided into multiple batches and each batch is inserted by a single transaction.
 
 ### tidb_batch_delete
 
 - Scope: SESSION | GLOBAL
 - Default value: 0
-- This variable is used to set whether to divide the data for deletion automatically.
-- When deleting a large amount of data, you can set the variable value to true. Then the data for deletion is automatically divided into multiple batches and each batch is deleted by a single transaction.  
+- This variable is used to set whether to divide the data for deletion automatically. It is valid only when `autocommit` is enabled.
+- When deleting a large amount of data, you can set the variable value to true. Then the data for deletion is automatically divided into multiple batches and each batch is deleted by a single transaction.
 
 ### tidb_dml_batch_size