doc: describe child.kill() pitfalls on linux

This commit refines the documentation around child.kill(), where kill
attempts against shells will lead to unexpected results. Namely, on
linux the child process of a child process will not terminate, when
its parent gets terminated. This is different across the the
platforms.

PR-URL: https://github.com/nodejs/node/issues/2098


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Closes: https://github.com/nodejs/node/issues/2098