The "latex" writer provides the (provisional) setting "use_bibtex" to defer
the resolution of citation references to the backend (LaTeX/BibTeX/LaTeX).
Use the `unknown_reference_resolvers` hook of the `TransformSpec` to
mark citation references as resolved if BibTeX is used.
Advantages:
* replaces hard-coded special-case in
`transforms.references.DanglingReferencesVisitor` introduced in [r9312],
* provides a usage example,
* tests the hook with the Docutils test suite without additional overhead.
Update docstrings and comments.