Improving doc_tests

Document tests, examples written into the documentation for structs, methods, and modules, do a great job of explaining how one might use the associated code. They are also somewhat rare in the code at the moment. Occasionally this is because the associated code is terrifying and shouldn't be used, but when this isn't the case (or shouldn't be the case), we can improve the situation with better examples.

Document test PRs are much loved, especially ones that demonstrate the fundamental features of the code with some sort of test at the end showing what should be true about the result or behavior.

No files, tests, or entry points are named. Survey the Rust structs, methods, and modules whose documentation lacks examples, starting with code that is intended for normal use rather than code described as terrifying. Done means adding documentation examples with a test at the end that demonstrates the expected result or behavior.