The description of `tidb_build_stats_concurrency` in the documentation is incorrect

[Rustin170506]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

Check https://docs.pingcap.com/zh/tidb/dev/statistics#tidb_build_stats_concurrency

2. What did you expect to see? (Required)

It should explain that it controls concurrency at the table or partition level.

3. What did you see instead (Required)

It says that this parameter controls the concurrency at the column level.

4. What is your TiDB version? (Required)

dev

[Rustin170506]

The description of tidb_analyze_partition_concurrency is also incorrect. It only controls the write operations.

[Rustin170506]

Also, it seems we do not use tidb_index_serial_scan_concurrency anymore.

[dveeden]

Maybe this should have gone to https://github.com/pingcap/docs/ and/or https://github.com/pingcap/docs-cn/ That's also where you could create a PR if you want

[dveeden]

cc @Oreoxmt @qiancai