Add more detail to yaml test descriptions

[Gabeblis]

Committer Notes

Description

Added more detail to yaml test descriptions. These changes should be added as they are part of the effort write all of the constraints from the constraint tracker.

Changes

• Modified yaml tests to add more detail to the test description.
• Modified feature-external-constraints.xml to change has-data-flow-diagram level from ERROR to WARNING (correction).

All Submissions:

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• Have you squashed any non-relevant commits and commit messages? [instructions]
• Have you added an explanation of what your changes do and why you'd like us to include them?
  - [ ] If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
  - [ ] If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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