tidb: update tidb_index_serial_scan_concurrency docs (#21938)

statistics.md

@@ -407,15 +407,20 @@ The relationships of the relevant system variables are shown below:
 
 #### `tidb_build_stats_concurrency`
 
-When you run the `ANALYZE` statement, the task is divided into multiple small tasks. Each task only works on statistics of one column or index. You can use the [`tidb_build_stats_concurrency`](/system-variables.md#tidb_build_stats_concurrency) variable to control the number of simultaneous small tasks. The default value is `2`. The default value is `4` for v7.4.0 and earlier versions.
+This variable controls the concurrency for building statistics during manual `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously. The default value is `2`. The default value is `4` for v7.4.0 and earlier versions.
 
 #### `tidb_build_sampling_stats_concurrency`
 
-When analyzing ordinary columns, you can use [`tidb_build_sampling_stats_concurrency`](/system-variables.md#tidb_build_sampling_stats_concurrency-new-in-v750) to control the concurrency of executing sampling tasks. The default value is `2`.
+This variable controls the following aspects of `ANALYZE` concurrency:
+
+- The concurrency for merging samples collected from different Regions.
+- The concurrency for collecting statistics on special indexes (such as indexes on generated virtual columns), for example, the number of indexes that TiDB can concurrently collect statistics for.
+
+The default value is `2`.
 
 #### `tidb_analyze_partition_concurrency`
 
-When running the `ANALYZE` statement, you can use [`tidb_analyze_partition_concurrency`](/system-variables.md#tidb_analyze_partition_concurrency) to control the concurrency of reading and writing statistics for a partitioned table. The default value is `2`. The default value is `1` for v7.4.0 and earlier versions.
+This variable controls the concurrency for saving `ANALYZE` results (writing TopN and histograms to system tables). The default value is `2`. The default value is `1` for v7.4.0 and earlier versions.
 
 #### `tidb_distsql_scan_concurrency`
 

system-variables.md

@@ -1092,7 +1092,7 @@ MPP is a distributed computing framework provided by the TiFlash engine, which a
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Default value: `2`. The default value is `1` for v7.4.0 and earlier versions.
-- This variable specifies the concurrency of reading and writing statistics for a partitioned table when TiDB analyzes the partitioned table.
+- For manual and auto `ANALYZE`, this variable controls the concurrency for saving `ANALYZE` results, including writing TopN and histograms to system tables.
 
 ### tidb_analyze_version <span class="version-mark">New in v5.1.0</span>
 
@@ -1214,7 +1214,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Type: Integer
 - Default value: `1`
 - Range: `[1, 256]`
-- This variable is used to set the concurrency of executing the automatic update of statistics.
+- This variable controls the concurrency for building statistics during auto `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously.
 
 ### tidb_backoff_lock_fast
 
@@ -1319,8 +1319,7 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Default value: `2`. The default value is `4` for v7.4.0 and earlier versions.
 - Range: `[1, 256]`
 - Unit: Threads
-- This variable is used to set the concurrency of executing the `ANALYZE` statement.
-- When the variable is set to a larger value, the execution performance of other queries is affected.
+- This variable controls the concurrency for building statistics during manual `ANALYZE`, such as the number of table or partition analysis tasks that can be processed simultaneously.
 
 ### tidb_build_sampling_stats_concurrency <span class="version-mark">New in v7.5.0</span>
 
@@ -1331,8 +1330,9 @@ mysql> SELECT job_info FROM mysql.analyze_jobs ORDER BY end_time DESC LIMIT 1;
 - Unit: Threads
 - Default value：`2`
 - Range: `[1, 256]`
-- This variable is used to set the sampling concurrency in the `ANALYZE` process.
-- When the variable is set to a larger value, the execution performance of other queries is affected.
+- This variable controls the following aspects of `ANALYZE` concurrency:
+    - The concurrency for merging samples collected from different Regions.
+    - The concurrency for collecting statistics on special indexes (such as indexes on generated virtual columns), for example, the number of indexes that TiDB can concurrently collect statistics for.
 
 ### tidb_capture_plan_baselines <span class="version-mark">New in v4.0</span>
 
@@ -3663,7 +3663,7 @@ For a system upgraded to v5.0 from an earlier version, if you have not modified
 - Persists to cluster: Yes
 - Applies to hint [SET_VAR](/optimizer-hints.md#set_varvar_namevar_value): No
 - Default value: `1`
-- This variable specifies the concurrency of merging statistics for a partitioned table when TiDB analyzes the partitioned table.
+- This variable controls the concurrency for merging TopN results of partitioned tables.
 
 ### tidb_enable_async_merge_global_stats <span class="version-mark">New in v7.5.0</span>