doc: refine child_process detach behaviour

this adds an example of a long running node process that actually
executes node code.
Also it mentions the not to harmonic detach behaviours of the
different platforms, whereas detaching on unix requires ignoring
the child_process' stdio explicitely.

PR-URL: https://github.com/nodejs/node/pull/5330


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>