doc: edit n-api.md for minor improvements

Not a comprehensive edit. Just a few things I noticed.

* Favor present tense where there's a choice
* Added a comma for clarity
* Added a missing word ("in")

PR-URL: https://github.com/nodejs/node/pull/35361


Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>