docs: clarify utilityProcess child.pid value (#44339)

docs/api/utility-process.md

@@ -86,9 +86,21 @@ true if the kill is successful, and false otherwise.
 #### `child.pid`
 
 A `Integer | undefined` representing the process identifier (PID) of the child process.
-If the child process fails to spawn due to errors, then the value is `undefined`. When
+Until the child process has spawned successfully, the value is `undefined`. When
 the child process exits, then the value is `undefined` after the `exit` event is emitted.
 
+```js
+const child = utilityProcess.fork(path.join(__dirname, 'test.js'))
+
+child.on('spawn', () => {
+  console.log(child.pid) // Integer
+})
+
+child.on('exit', () => {
+  console.log(child.pid) // undefined
+})
+```
+
 #### `child.stdout`
 
 A `NodeJS.ReadableStream | null` that represents the child process's stdout.