src: describe what NODE_MODULE_VERSION is for

Current comment described what to do with it when the ABI changes, but
implied that Node.js would load modules with newer ABI numbers, which it
will not.

PR-URL: https://github.com/nodejs/node/pull/10414


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>