doc: make references clickable

The child_process docs contained some links, but some links were
somehow "broken". `make doc` couldn't translate them and left `[]`
in the compiled HTML.

PR-URL: https://github.com/nodejs/node/pull/4654


Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>