[Dev-luatex] `os.execute` special output is still undocumented

The thread https://tug.org/pipermail/luatex/2015-November/005536.html points out that `os.execute` is still version `5.1`. It is said there that "one can always wrap os.execute into something that does what one wants" How to do that to get the "5.3" output of `os.execute`, in any situation of course? This design policy is so special that it should definitely be documented.