Improving documentation

This is a long-term improvement issue, to identify and improve lacking documentation. Although timely has #deny[missing_docs], this doesn't mean that the documentation is uniformly good. Several structs and methods could have clearer explanations of their roles and functionality, and several modules could have better explanations of their organization and principles.

Document improvement PRs are great and loved, especially with an eye towards increased clarity (if not necessarily increased word count).

Start by surveying the repository's Rust structs, methods, and modules for unclear or missing explanations, using the existing documentation and the deny[missing_docs] lint as context. Choose a focused area, then improve its descriptions and organization principles; done means the selected documentation is clearer without unnecessary expansion.