doc: add process.debugPort to doc/api/process.md

Fixes: https://github.com/nodejs/node/issues/18639

PR-URL: https://github.com/nodejs/node/pull/18716
Refs: https://github.com/nodejs/node/issues/18639


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matheus Marchini <matheus@sthima.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>