Clarify specification for processing combined allowed-value constraints

[aj-stein-nist]

User Story:

As a Metaschema developer, in order to understand how explain, design, and implement usage of Metaschema-based models, I need an updates to the specification on the Metaschema website re enumerated values, especially when multiple constraints constrain the same elements in instances of that model.

See usnistgov/OSCAL-Reference#6 for surrounding context. We need this clarification to unblock this documentation fix request.

Goals:

• Confirm current expected behavior
• Document current expected behavior

Dependencies:

N/A

Acceptance Criteria

• All website and readme documentation affected by the changes in this issue have been updated. Changes to the website can be made in the docs/content directory of your branch.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.
• The CI-CD build process runs without any reported errors on the PR. This can be confirmed by reviewing that all checks have passed in the PR.

{The items above are general acceptance criteria for all User Stories. Please describe anything else that must be completed for this issue to be considered resolved.}