Add clarification around collection-level assets in item-assets.

[lossyrob]

This PR adds language to the item-assets spec to clarify that collection-level asset objects should not contain properties that are not explicitly stated in the item's asset objects themselves.

This comes out of a discussion during STAC Sprint 6. It was determined that while allowing for asset properties to be defined in only the collection-level asset objects was a way to save space, it would be more in line with the spec to have items explicitly contain all asset property information.

If you're currently using the item-assets extension to consolidate item-level asset object properties in an API setting, consider storing the STAC items without those consolidated properties internally as 'invalid' STAC items, and merge in the desired collection-level asset object properties at serving time.

PR Checklist:

• This PR is made against the dev branch (all proposed changes except releases should be against dev, not master).
• This PR has no breaking changes.
• I have added my changes to the CHANGELOG or a CHANGELOG entry is not required.
• This PR affects the STAC API spec, and I have opened issue/PR #XXX to track the change.