More extensive documentation for step advance

Step advance has come a long way it the past month or two, to the point that we should probably start documenting best practices in the guide. This will probably be a long running task since we're still discovering what works best for different scenarios, but we can start at a very high level with some things that won't easily be obsoleted.

In parallel and more immediately, we can probably take a refresh pass over the API docs that get published on docs.rs to add more examples. These are great because they are easy to link to and don't suffer from code rot (they run as part of the automated tests :D).

Start by locating the step advance guide and the API documentation published on docs.rs. Document stable, high-level best practices and add examples that are covered by automated tests; the issue does not name specific files or tests, so confirm those entry points first.

Written by the indexing model from the issue text.