doc: cleanup n-api.md doc

Partial doc cleanup as per
https://github.com/nodejs/node/issues/20421

PR-URL: https://github.com/nodejs/node/pull/20430


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>