doc: clarify information about ABI version

PR-URL: https://github.com/nodejs/node/pull/10419


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Refs: https://github.com/nodejs/node/pull/9901#discussion_r93575704