Clarify bbox and geometry field documentation

[duckontheweb]

The current "bbox and geometry" section of the README seems pretty confusing.

• ml-aoi Multiple items may reference the same label and image item by scoping the bbox and geometry fields. TODO: Better describe scoping of overlap between raster and label items?

We should provide a more thorough description of what this means along with some examples

ml-aoi Items in the same Collection should never have overlapping geometry fields.

Is this really a requirement? What if we had a label + source imagery pair from a scene taken on 2021-01-01 and then another label + source imagery pair from a scene taken on 2021-01-02 over the same area? It seems like those would represent valid and distinct ML AOI Items with overlapping geometries.

• ml-aoi Items bbox field may overlap when they belong to different ml-aoi:split set.

The same logic would apply here as well; it seems there could be valid cases where we have Items in the same split with overlapping bboxes.