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

Fixes: nodejs#18639

PR-URL: nodejs#18716
Refs: nodejs#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>

doc/api/process.md

@@ -631,7 +631,17 @@ process.
 ```js
 console.log(`Current directory: ${process.cwd()}`);
 ```
+## process.debugPort
+<!-- YAML
+added: v0.7.2
+-->
+* {number}
 
+The port used by Node.js's debugger when enabled.
+
+```js
+process.debugPort = 5858;
+```
 ## process.disconnect()
 <!-- YAML
 added: v0.7.2