doc: napi: make header style consistent

Some of the API section headers had emphasis, while others did not.
Use a consistent style (no emphasis) for all of them, as that matches
our other API docs.

PR-URL: nodejs#18122
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Kyle Farnung <[email protected]>
Reviewed-By: Jon Moss <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>
Reviewed-By: Tiancheng "Timothy" Gu <[email protected]>
Reviewed-By: Michael Dawson <[email protected]>

Author: ofrobots