doc: document the error when cwd not exists in child_process.spawn

If the option cwd does not exist, the error ENOENT is the same as
the error emitted when the command does not exist, it's confusing.

PR-URL: #34505
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

Author: FeelyChau

doc/api/child_process.md

@@ -469,7 +469,10 @@ const defaults = {
 ```
 
 Use `cwd` to specify the working directory from which the process is spawned.
-If not given, the default is to inherit the current working directory.
+If not given, the default is to inherit the current working directory. If given,
+but the path does not exist, the child process emits an `ENOENT` error
+and exits immediately. `ENOENT` is also emitted when the command
+does not exist.
 
 Use `env` to specify environment variables that will be visible to the new
 process, the default is [`process.env`][].