tikv: document backup.num-threads config (#4057)
tikv: document backup.num-threads config (#4057)

* tikv: document backup.num-threads config

Signed-off-by: kennytm <kennytm@gmail.com>

* Apply suggestions from code review

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