doc: add Note of options.stdio into child_process
add Note of options.stdio
into child_process.
Ref: https://github.com/nodejs/node/pull/22892#issuecomment-469173273
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines