Common metadata: Clarify in descriptions that the fields are not Item-only

[m-mohr]

Related Issue(s): None

Proposed Changes:

1. Common metadata: Clarify in various field descriptions that the fields do not only apply to Items

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.

[constantinius]

This clarifies the usage of common metadata fields to not only apply to Items. Is there perhaps a chance to put this document into a more central location (like common/)? Or at least add links from the catalog and collection specs?

[m-mohr]

I prefer the links from catalog/collection, as many other documents outside of the repo point to the file and we don't want to break these.