Add doc for tidb_executor_concurrency (#3975)
Add doc for tidb_executor_concurrency (#3975)

* add executor_concurrency doc

Signed-off-by: niedhui <1a3ec98be0900d05adba70f95db19f4c8e6a2f32@gmail.com>

* add doc for deprecated varialbes

Signed-off-by: niedhui <1a3ec98be0900d05adba70f95db19f4c8e6a2f32@gmail.com>

* try fix link

Signed-off-by: niedhui <1a3ec98be0900d05adba70f95db19f4c8e6a2f32@gmail.com>

* correct note format

* add back initial changes

* Correct the format of multiple notes

* refine language

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
Co-authored-by: Lilian Lee <c684465c17314189c18707959627e2ec4fdec2d0@pingcap.com>
pingcap/docs-cn,pingcap/docs-cn