doc: clarify child_process.exec inherits cwd

PR-URL: https://github.com/nodejs/node/pull/36809


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Pooja D P <Pooja.D.P@ibm.com>
Reviewed-By: Yash Ladha <yash@yashladha.in>
Reviewed-By: James M Snell <jasnell@gmail.com>