updates from PR review

overview.md

@@ -21,18 +21,17 @@ JSON Schemas provide the core testing definitions, and are used in an array of v
 on [RFC 8288 (Web Linking)](https://tools.ietf.org/rfc/rfc8288.txt) to express relationships between resources,
 and IANA [Media Types](https://en.wikipedia.org/wiki/Media_type) to describe file formats and format contents.
 The [OGC API - Features](https://ogcapi.ogc.org/features/) standard is a final core building block. The STAC
-Collection extends their [Feature Collection](http://docs.opengeospatial.org/is/17-069r3/17-069r3.html#_collection_)
-(and the full API is the foundation for the STAC API specification).
+Collection extends their [Collection](http://docs.opengeospatial.org/is/17-069r3/17-069r3.html#_collection_)
+JSON (and the full API is the foundation for the STAC API specification).
 
 The STAC specifications are written to be understandable without needing a full background in these. But if you 
 want to get deep into STAC tool implementation and are not familiar with any of the standards mentioned above it is 
 recommended to read up on them. STAC development is guided by set of core philosophical tenets, like 
 building small reusable parts that are loosely coupled, focusing on developers, and more - see our the 
 [principles](principles.md) document to learn more.
 
-*Note: Setting a field in JSON to `null` is not equivalent to a field not appearing. STAC defines `null` explicitly 
-for some fields, where it has a particular meaning. It should not be used otherwise - instead the field should be left 
-out entirely.* 
+*Note: Setting a field in JSON to `null` is not equivalent to a field not appearing in STAC, as JSON Schema tools treat
+them differently. STAC defines `null` explicitly for some fields, where it has a particular meaning. So `null` should not be used unless the STAC spec defines its use - instead the field should be left out entirely.* 
 
 ## Item Overview