Update necessary description (#4645)
Update necessary description (#4645)

* Update necessary description

Explain the default value as necessary

* Update role-based-access-control.md

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