n-api: experimental napi_add_finalizer documented as stable api

* **Version**: 10.13.0
* **Platform**: all
* **Subsystem**: n-api

`napi_add_finalizer` is declared as experimental in `node_api.h`([here](https://github.com/nodejs/node/blob/v10.13.0/src/node_api.h#L698)), yet it does not been documented as experimental in [document](https://nodejs.org/docs/latest-v10.x/api/n-api.html#n_api_napi_add_finalizer).