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: https://github.com/nodejs/node/pull/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>