Clearly state where the fields can be used

[m-mohr]

Currently we often have headers such as "Item properties and Collection Field" although often field can be used in a variety of additonal places (e.g. Assets)

Wouldn't it be more specific and easier to adopt the template to write something instead:

## Fields

The following fields can be used in the following parts of a STAC document:
- [ ] Catalogs
- [x] Collections
- [x] Item Properties (incl. Summaries in Collections)
- [x] Assets (both Collections and Items, incl. Item Asset Definitions in Collections)
- [ ] Links

....

Example: https://github.com/stac-extensions/datacube