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: https://github.com/nodejs/node/pull/34505


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