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), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines