tikv: update backup.num-threads configuration (#3442)
tikv: update backup.num-threads configuration (#3442)

* tikv: document backup.num-threads config

* Update tikv-configuration-file.md

* Update tikv-configuration-file.md

* Apply suggestions from code review

Co-authored-by: kennytm <d10ee62456d8e2be8715605d23b69b7662d1915a@gmail.com>

* Update tikv-configuration-file.md

Co-authored-by: kennytm <d10ee62456d8e2be8715605d23b69b7662d1915a@gmail.com>

Co-authored-by: kennytm <d10ee62456d8e2be8715605d23b69b7662d1915a@gmail.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs