doc: child_process: pid can be undefined when ENOENT

From the code `nodejs@8` and up should behave the same: https://github.com/nodejs/node/blame/v8.17.0/lib/internal/child_process.js#L290-L316

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>

doc/api/child_process.md

@@ -1255,7 +1255,9 @@ added: v0.1.90
 
 * {integer}
 
-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 like command or `cwd` not found (`ENOENT`), then
+the value is `undefined` and `error` should emit on next tick.
 
 ```js
 const { spawn } = require('child_process');