Docs: Document distance type and sort mode for many to many geo_points

Can you  mention that `plane` might be completely wrong for long distances or close to the poles? I also think the wording is not totally fair to `sloppy` since it is actually almost as precise as `arc`? What about something like

```
How to compute the distance. Can either be `sloppy_arc` (default), `arc` (slighly more precise but significantly slower) or `plane` (faster, but inaccurate on long distances and close to the poles).
```