doc: clarify conditional exports guidance

PR-URL: https://github.com/nodejs/node/pull/34306


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Jan Krems <jan.krems@gmail.com>