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, which might also invite some core work.
I felt this to be necessary after researching for #5146 (closed), whereas the the behaviour of detaching seems highly inconsistent and very untransparent for the user.
/cc @nodejs/documentation