[Packwerk 3.0] Ensure API boundaries are up-to-date

[alexevanczuk]

We want to use this major version release to take the opportunity to mark undocumented API as explicitly private, using any combination of:

• private
• private_class_method
• private_constant
• # :node or whatever YARD uses

Similarly, we'll want to make sure all public API has clear, intentional, and documented interfaces.

[gmcgibbon]

This looks good to me since your changes in #276, #285, and #289. We may want to add more documentation, but I think we can do that going forward. Documentation PRs are welcome!