doc: clarify esm conditional exports prose

This commit clarifies the behavior of a couple aspects of
conditional exports that may have been difficult to grasp
from the prose alone.

PR-URL: https://github.com/nodejs/node/pull/33886


Reviewed-By: Guy Bedford <guybedford@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>