Updates on model namespacing & uniqueness

[jtcohen6]

resolves #3571

What are you changing in this pull request and why?

Starting in v1.6, model names can be duplicated across different namespaces (packages/projects). They must still be unique within one package/project.

This requires updates to:

• our FAQ on model name uniqueness
• two-argument ref
• note on snapshot name uniqueness
• (I'm sure I've missed some places)

Checklist

• Add versioning components, as described in Versioning Docs
• Add a note to the prerelease version Migration Guide
• Review the Content style guide and About versioning so my content adheres to these guidelines.
• Add a checklist item for anything that needs to happen before this PR is merged, such as "needs technical review" or "change base branch."

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[dbeatty10]

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