doc: clarify subprocess.stdout/in/err/io properties

The various `subprocess.stdwhatever` properties can be null or
undefined in certain conditions.

Fixes: #33458
Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: #36784
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: jasnell

doc/api/child_process.md

@@ -1498,6 +1498,9 @@ 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
+not be successfully spawned.
+
 ### `subprocess.stdin`
 <!-- YAML
 added: v0.1.90
@@ -1516,6 +1519,9 @@ 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
+not be successfully spawned.
+
 ### `subprocess.stdio`
 <!-- YAML
 added: v0.7.10
@@ -1556,6 +1562,9 @@ assert.strictEqual(subprocess.stdio[2], null);
 assert.strictEqual(subprocess.stdio[2], subprocess.stderr);
 ```
 
+The `subprocess.stdio` property can be `undefined` if the child process could
+not be successfully spawned.
+
 ### `subprocess.stdout`
 <!-- YAML
 added: v0.1.90
@@ -1581,6 +1590,9 @@ subprocess.stdout.on('data', (data) => {
 });
 ```
 
+The `subprocess.stdout` property can be `null` if the child process could
+not be successfully spawned.
+
 ### `subprocess.unref()`
 <!-- YAML
 added: v0.7.10