update: doc/api/child_process.md

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

Author: dr-js

doc/api/child_process.md

@@ -1253,11 +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. 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.
+fails to spawn due to errors, then
+the value is `undefined` and `error` is emitted.
 
 ```js
 const { spawn } = require('child_process');