doc: clarify subprocess.stdout/in/err property

doc/api/child_process.md

@@ -1680,7 +1680,7 @@ then this will be `null`.
 `subprocess.stderr` is an alias for `subprocess.stdio[2]`. Both properties will
 refer to the same value.
 
-The `subprocess.stderr` property can be `null` if the child process could
+The `subprocess.stderr` property can be `null` or `undefined` if the child process could
 not be successfully spawned.
 
 ### `subprocess.stdin`
@@ -1702,7 +1702,7 @@ then this will be `null`.
 `subprocess.stdin` is an alias for `subprocess.stdio[0]`. Both properties will
 refer to the same value.
 
-The `subprocess.stdin` property can be `undefined` if the child process could
+The `subprocess.stdin` property can be `null` or `undefined` if the child process could
 not be successfully spawned.
 
 ### `subprocess.stdio`
@@ -1775,7 +1775,7 @@ subprocess.stdout.on('data', (data) => {
 });
 ```
 
-The `subprocess.stdout` property can be `null` if the child process could
+The `subprocess.stdout` property can be `null` or `undefined` if the child process could
 not be successfully spawned.
 
 ### `subprocess.unref()`