doc: document the fds behind stdin/out/err

Its common knowledge on unix, but node documentation depends on knowing
this, as it exposes both streams named after stdio, and the fd numbers,
so make this explicit.

Fixes: https://github.com/joyent/node/pull/8624
PR-URL: https://github.com/joyent/node/pull/8454


Reviewed-by: Trevor Norris <trev.norris@gmail.com>