doc: cover stdio option in child_process

- Add hyperlinks from spawn options to subsections detailing what
those options do.
- Clarify some verbiage around ChildProcess.prototype.std{in,out,err}.
- Remove second-person pronoun.

PR-URL: https://github.com/joyent/node/pull/8639


Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>