doc: add reference to node_api.h in docs

Realized that we don't actually point people to the file to
include in order to access N-API functions.  Add that.

PR-URL: https://github.com/nodejs/node/pull/13084


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>