Border around or over the shape

Documentation Is:

- [ ] Missing or needed
- [x] Confusing
- [ ] Not Sure?

### Please Explain in Detail...
Are borders in ChartJs generally meant to be:

- drawing around a shapes background (leads to bigger size of the shape)
- drawing over a shapes background (leads to same size of a shape)

In the following doughnut chart I tried to add a transparent border, which isn't working as the border seems to be drawn over the shape:
https://codepen.io/elitastic/pen/BadJePW

I'm not sure if this is a bug or by design. However, the bar chart behaves differently, the border is drawn around the bar: https://codepen.io/elitastic/pen/RwZxzNo

I've a use case, where I should have a transparent border in a doughnut chart, white borders are not working cause of a dynamic background:
https://codepen.io/elitastic/pen/BadJePW

### Your Proposal for Changes
I would expect the border to be drawn around the shape like in the bar chart. Either way it would great if this would be documented.