doc: improve napi formatting

This addresses two nits:

1. Bullet points that span over multiple lines should be indented
   consitently. Otherwise they are not always properly highlighted.
2. Lines breaks after titles improve the readability.

PR-URL: https://github.com/nodejs/node/pull/30772


Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>