doc: child_process documentation for stdio
Checklist
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documentation is changed or added -
commit message follows commit guidelines
Affected core subsystem(s)
doc
Description of change
Document that execFileSync, execSync and spawnSync also supports stdio as an Array.
execFileSync, execSync and spawnSync all ends up here, where the options are accepted as string or array. See https://github.com/nodejs/node/blob/master/lib/internal/child_process.js#L758