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!
Would it be OK to mention that Symfony does this for performance reasons?