Add style guide requirements re context item operator, relative, and absolute path expression

[aj-stein-gsa]

This is a ...

improvement - something could be better

This relates to ...

• the FedRAMP OSCAL Validations

User Story

As a developer of FedRAMP OSCAL constraints, in order to increase clarity and decrease ambiguity regarding absolute and relative paths used in @target and @test for individual constraints and their //context/metapath/@target, I would like clear guidance for developers on how to use /absolute-reference-to-top-level-model and .//relative/recursive/path/to/child/model/element accordingly.

Goals

• Document the effective use absolute and relative path the context item operator, absolute, and relative path expressions with a new Constraint Style Guide recommendation (note it is a recommendation, not a requirement; this is a "SHOULD", not a "MUST")

Dependencies

No response

Acceptance Criteria

• All FedRAMP Documents Related to OSCAL Adoption (https://github.com/GSA/fedramp-automation) affected by the changes in this issue have been updated.
• A Pull Request (PR) is submitted that fully addresses the goals of this User Story. This issue is referenced in the PR.

Other information

Developers in the team identified well-formed and valid Metapath syntax but confusing behavior regarding relative and absolute path behavior without the . context item operator.

[aj-stein-gsa]

This is definitely something that is worth working on if we have the bandwidth this week or next (we likely will have that), but I need to implement snapshot release support for metaschema-java, liboscal-java, and/or oscal-cli per metaschema-framework/metaschema-java#349 to make this workable before an updated oscal-cli release (which is also in the works, but slow moving). Will mark this as blocked but in scope to wrap the DAP Phase 1 of the pilot.