fix: utilityProcess pid should be undefined after exit (#44693)

fix: utilityProcess pid should be undefined after exit

Co-authored-by: trop[bot] <37223003+trop[bot]@users.noreply.github.com>
Co-authored-by: Shelley Vohr <shelley.vohr@gmail.com>

docs/api/utility-process.md

@@ -92,6 +92,8 @@ the child process exits, then the value is `undefined` after the `exit` event is
 ```js
 const child = utilityProcess.fork(path.join(__dirname, 'test.js'))
 
+console.log(child.pid) // undefined
+
 child.on('spawn', () => {
   console.log(child.pid) // Integer
 })
@@ -101,6 +103,8 @@ child.on('exit', () => {
 })
 ```
 
+**Note:** You can use the `pid` to determine if the process is currently running.
+
 #### `child.stdout`
 
 A `NodeJS.ReadableStream | null` that represents the child process's stdout.