doc: add ref to option to enable n-api

Since its guarded in by a command line option say
that in the docs and provide the option that needs
to be used to enable it.

PR-URL: nodejs#13406
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießn <tniessen@tnie.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>

doc/api/n-api.md

@@ -60,6 +60,14 @@ For example:
 #include <node_api.h>
 ```
 
+As the feature is experimental it must be enabled with the
+following command line
+[option](https://nodejs.org/dist/latest-v8.x/docs/api/cli.html#cli_napi_modules):
+
+```bash
+--napi-modules
+```
+
 ## Basic N-API Data Types
 
 N-API exposes the following fundamental datatypes as abstractions that are