doc: add warning about Windows process groups

This commit adds a warning for Windows platforms. `process.kill` wont
kill a process group on Windows and instead it throws an error.

Refs: https://github.com/nodejs/node/issues/3617
PR-URL: https://github.com/nodejs/node/pull/3681


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>