COSMETIC: Attributes: Clarify attribute spec on several key points, and align it with the implementation definitions

[bleggett]

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Proposed Changes

• More completely and exhaustively define "what an attribute is", using the descriptors we actually use in OpenTDF, and explicitly state important implications of the definitions.

Checklist

• A clear description of the change has been included in this PR.
• A clear description of whether this change is a Major, Minor, Patch or cosmetic change as per the Versioning Guidelines has been included in this PR.
• All schema validation tests have been updated appropriately and are passing.
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• MAJOR/MINOR VERSION CHANGES ONLY: A link to a reference implementation (PR or set of PRs) of the change has been included in this PR.
• MAJOR/MINOR VERSION CHANGES ONLY: A writeup has been included discussing the motivation and impact of this change.
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• DRAFT MERGE ONLY: Tagged this branch with new semver version and an annotation describing the change (ex: git tag -s 4.1.0 -m "Spec version 4.1.0 - did a thing")
• DRAFT MERGE ONLY: Version numbers have been updated as per the Versioning Guidelines.
• This change otherwise adheres to the project Contribution Guidelines.

[stephaniegraham]

Looks great, thanks for responding to my nit-picks!