Clarify use of "Feature" in GeoJSON result response

[m-mohr]

A question that recently came up:

I have a doubt about the response of /jobs/{job_id}/results
the field type, can it be only Feature?
I set it to raster in my response, and the validator complains
but then it is a hard-coded value?

We need to clarify that the "Feature" is there because it's GeoJSON and only describes that the spatial reference (geometry) in the metadata is a feature, it doesn't describe what type the assets are.

cc @lforesta @bgoesswe

[m-mohr]

@lforesta I added the following clarification in the commit above:

        The GeoJSON type that applies to this metadata document,
        which MUST always be a "Feature" according to the STAC specification.

        This type does **not** describe the spatial data type of the assets.

Please let me know if there's anything that can still be improved.