Update note on collection, removed type

spec/stac-geoparquet-spec.md

@@ -27,7 +27,7 @@ most of the fields should be the same in STAC and in GeoParquet.
 | bbox               | Struct of Floats     | Required | Can be a 4 or 6 value struct, depending on dimension of the data                                                               |
 | links              | List of Link structs | Required | See [Link Struct](#link-struct) for more info                                                                                  |
 | assets             | An Assets struct     | Required | See [Asset Struct](#asset-struct) for more info                                                                                |
-| collection         | String               | Required | The ID of the collection this Item is a part of                                                                                |
+| collection         | String               | Optional | The ID of the collection this Item is a part of. See notes below on 'Collection' and 'Collection JSON' in the Parquet metadata |
 | *property columns* | *varies*             | -        | Each property should use the relevant Parquet type, and be pulled out of the properties object to be a top-level Parquet field |
 
 * Must be valid GeoParquet, with proper metadata. Ideally the geometry types are defined and as narrow as possible.
@@ -85,4 +85,4 @@ The principles here can likely be used to map into other geospatial data formats
 [link]: https://github.com/radiantearth/stac-spec/blob/master/item-spec/item-spec.md#link-object
 [common-media-types]: https://github.com/radiantearth/stac-spec/blob/master/best-practices.md#common-media-types-in-stac
 [timestamp]: https://github.com/apache/parquet-format/blob/master/LogicalTypes.md#timestamp
-[parquet-metadata]: https://github.com/apache/parquet-format#metadata
+[parquet-metadata]: https://github.com/apache/parquet-format#metadata