Add explaination about tidb_allow_batch_cop (#3002)

Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

tidb-specific-system-variables.md

@@ -571,3 +571,13 @@ set tidb_query_log_max_len = 20
 - Scope: SESSION
 - Default value: 0
 - This variable is used to show whether the execution plan used in the previous `execute` statement is taken directly from the plan cache.
+
+### tidb_allow_batch_cop <span class="version-mark">New in v4.0 version</span>
+
+- Scope: SESSION | GLOBAL
+- Default value: 0
+- This variable is used to control how TiDB sends a coprocessor request to TiFlash. It has the following values:
+
+    * `0`: Never send requests in batches
+    * `1`: Aggregation and join requests are sent in batches
+    * `2`: All coprocessor requests are sent in batches

tiflash/tune-tiflash-performance.md

@@ -23,10 +23,34 @@ If you want to save machine resources and have no requirement on isolation, you
     set @@tidb_distsql_scan_concurrency = 80;
     ```
 
-2. Enable the optimization for TiDB Operator such as the aggregate pushdown of `JOIN` or `UNION`:
+2. Enable the super batch feature:
+
+    You can use the [`tidb_allow_batch_cop`](/tidb-specific-system-variables.md#tidb_allow_batch_cop) variable to set whether to merge Region requests when reading from TiFlash.
+
+    When the number of Regions involved in the query is relatively large, try to set this variable to `1` (effective for coprocessor requests with `aggregation` operators that are pushed down to TiFlash), or set this variable to `2` (effective for all coprocessor requests that are pushed down to TiFlash).
+
+    {{< copyable "sql" >}}
+
+    ```sql
+    set @@tidb_allow_batch_cop = 1;
+    ```
+
+3. Enable the optimization of pushing down aggregate functions before TiDB operators such as `JOIN` or `UNION`:
+
+    You can use the [`tidb_opt_agg_push_down`](/tidb-specific-system-variables.md#tidb_opt_agg_push_down) variable to control the optimizer to execute this optimization. When the aggregate operations are quite slow in the query, try to set this variable to `1`.
 
     {{< copyable "sql" >}}
 
     ```sql
     set @@tidb_opt_agg_push_down = 1;
     ```
+
+4. Enable the optimization of pushing down aggregate functions with `Distinct` before TiDB operators such as `JOIN` or `UNION`:
+
+    You can use the [`tidb_opt_distinct_agg_push_down`](/tidb-specific-system-variables.md#tidb_opt_distinct_agg_push_down) variable to control the optimizer to execute this optimization. When the aggregate operations with `Distinct` are quite slow in the query, try to set this variable to `1`.
+
+    {{< copyable "sql" >}}
+
+    ```sql
+    set @@tidb_opt_distinct_agg_push_down = 1;
+    ```