Distinguish vacuum BC/zero-importance cell

[tjlaboss]

Pull Request Checklist for MontePy

Description

Minor boundary condition clarifications in the documentation. Fixes #744

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General Checklist

• I have formatted my code with black version 25.

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LLM Disclosure

Were any large language models (LLM or "AI") used in to generate any of this code?

• Yes
  • Model(s) used:
• No

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Additional Notes for Reviewers

Ensure that:

• This PR fully addresses and resolves the referenced issue(s).
• The submitted code is consistent with the merge checklist outlined here.
• The PR covers all relevant aspects according to the development guidelines.
• 100% coverage of the patch is achieved, or justification for a variance is given.

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📚 Documentation preview 📚: https://montepy--887.org.readthedocs.build/en/887/