DM: update openapi spec (pingcap#8612)

* add source change

* add task part

* fix blank space

* add more note

* Apply suggestions from code review

Co-authored-by: sunzy <sunzy2@gmail.com>

* fix lint

* fix again

* fix lint

* Update dm/dm-open-api.md

* Update dm/dm-open-api.md

* updata install guide

* Apply suggestions from code review

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

* Apply suggestions from code review

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

* fix link

* Apply suggestions from code review

* Update dm/dm-open-api.md

Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>

* remove note

* adjust space

* fix description

Co-authored-by: sunzy <sunzy2@gmail.com>
Co-authored-by: TomShawn <41534398+TomShawn@users.noreply.github.com>