doc: clarify behavior of napi_extended_error_info

Fix up example and make it more explicit on how
you need to use napi_extended_error_info in order to
help people avoid what might be a common mistake that
we made in node-addon-api.

Refs: https://github.com/nodejs/node-addon-api/issues/1089



Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: https://github.com/nodejs/node/pull/40458


Reviewed-By: Gabriel Schulhof <gabrielschulhof@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>