doc: clarify msg when doc/api/cli.md not updated

PR-URL: https://github.com/nodejs/node/pull/10872


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>