doc,child_process: pid can be undefined when ENOENT

From the code `nodejs@8` and up should behave the same:
github.com/nodejs/node/blame/v8.17.0/lib/internal/child_process.js#L290

And a short test snippet:
```js
const { spawn } = require('child_process')

const subProcess = spawn('non-exist-command')
subProcess.on('error', (error) =>
	console.warn('mute Unhandled "error" event:', error))
console.log('- pid:', subProcess.pid)
process.nextTick(() => console.log('- after error emit'))
console.log('== end of test ==')
```

Note: the sync spawn result `pid` currently do not follow this pattern.

Co-authored-by: Rich Trott <rtrott@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/37014
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>

doc/api/child_process.md

@@ -1253,9 +1253,11 @@ does not indicate that the child process has been terminated.
 added: v0.1.90
 -->
 
-* {integer}
+* {integer|undefined}
 
-Returns the process identifier (PID) of the child process.
+Returns the process identifier (PID) of the child process. If the child process
+fails to spawn due to errors, then the value is `undefined` and `error` is
+emitted.
 
 ```js
 const { spawn } = require('child_process');