doc: clarify child_process promise rejections

Promisify sections of `child_process.exec()`
and `child_process.execFile()` changed to make clear
that non-zero exit codes will result in promise rejection.

PR-URL: https://github.com/nodejs/node/pull/19541
Fixes: https://github.com/nodejs/node/issues/19494


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>