doc: change "considered experimental" to "experimental" in n-api.md

This changes "considered" experimental" to "experimental" in the
n-api.md file and introduces some additional brevity.

PR-URL: https://github.com/nodejs/node/pull/34129


Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>