doc: fix out of date sections in n-api doc

PR-URL: https://github.com/nodejs/node/pull/13508
Fixes: https://github.com/nodejs/node/issues/13469
Fixes: https://github.com/nodejs/node/issues/13458


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jason Ginchereau <jasongin@microsoft.com>