Improve documentation and examples in `arti-client`

(This is a placeholder ticket, made so that people can find it when they search for things to do under the First Contribution label.)

Try to write a program using arti. (The interface in the arti-client crate is the place to start.)

As you do this, you'll probably find that the documentation didn't explain something you wanted to know, or didn't explain it very well. After you figure out the issue (either by asking us, or reading the code), why not contribute a patch to improve the documentation?

---

Also, it's a good convention for all Rust code to have rustdoc examples for how to use it. These examples can be at the function level, the module level, or the type level. Right now, a lot of our crates are missing those. (arti-client is most important, but examples everywhere are welcome.)

When writing examples, please make sure that the example actually shows people how they would would want to use the code, and what happens when they do.