doc: fix return types for sync methods

For the sync methods spawnSync(), execSync() and execFileSync(), when
the stdio option is set to anything other than 'pipe', the stdout and
stderr returned from the C++ code will be null, and not a string or
Buffer as currently documented.

PR-URL: https://github.com/nodejs/node/pull/58575
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>
Reviewed-By: Gürgün Dayıoğlu <hey@gurgun.day>
Reviewed-By: James M Snell <jasnell@gmail.com>