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: #19541
Fixes: #19494
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/child_process.md

@@ -223,8 +223,9 @@ the existing process and uses a shell to execute the command.
 
 If this method is invoked as its [`util.promisify()`][]ed version, it returns
 a Promise for an object with `stdout` and `stderr` properties. In case of an
-error, a rejected promise is returned, with the same `error` object given in the
-callback, but with an additional two properties `stdout` and `stderr`.
+error (including any error resulting in an exit code other than 0), a rejected
+promise is returned, with the same `error` object given in the callback, but
+with an additional two properties `stdout` and `stderr`.
 
 ```js
 const util = require('util');
@@ -301,7 +302,8 @@ encoding, `Buffer` objects will be passed to the callback instead.
 
 If this method is invoked as its [`util.promisify()`][]ed version, it returns
 a Promise for an object with `stdout` and `stderr` properties. In case of an
-error, a rejected promise is returned, with the same `error` object given in the
+error (including any error resulting in an exit code other than 0), a rejected
+promise is returned, with the same `error` object given in the
 callback, but with an additional two properties `stdout` and `stderr`.
 
 ```js