Tried to clarify private services

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | kind of
| Applies to | all
| Fixed tickets | #4524

It was a though one to describe, as I wanted to make it clear and short. I'm really happy if people can review this and provide other, better, alternatives!
I would change `*cannot*` to `**should not**` or even `**must not**`.