add inline code formatting and clarify importance of success callback

Two main things:

- `success` added to the top example. It's an important feature, and it wasn't mentioned until pretty far down the page.
- Clarified that `success` is the only option that is run after the object has `save!` called on it. I think that someone could have guessed that from the docs, it took me a bit to figure out.

I also added some inline code formatting to make it a bit easier to scan.