Improve documentation and add a few small features and fixes

[dralley]

I'm trying to split the irrelevant stuff out of my other PRs and get it merged a bit sooner, while I'm working out a few details on the more complex stuff. Making those PRs smaller is also nice.

The reason for all the little features is that the gaps were encountered in the process of writing new tests.

I went through some effort to make sure the commits are split cleanly as opposed to the other PRs where they got mashed up a bit.

📜 Checklist

• Commits are cleanly separated and have useful messages
• A changelog entry or entries has been added to CHANGELOG.md
• Documentation is thorough
• Test coverage is excellent and passes
• Works when tests are run --all-features enabled

[drahnr]

Since there is no rpm spec, I find it helpful to have references to the C code names.

[dralley]

I don't really want super direct references to the RPM source code if it can be helped

[drahnr]

A few nits:

• %foo should always be code, generally more inline code blocks would allow to use i.e. tools like cargo-spellcheck without too many false positives eventually
• inconsistent TODO vs @todo lines, we shouldn't have any but track them in issues

other than that,

LGTM 👍