Documentation touchup: markers and arithmetic#2

Superficial documentation improvements for three handlers: ## `markerHandler` & `runMarkersHandler` Split the documentation on `runMarkersHandler` across both it and `markerHandler` in a way that hopefully shows how they interoperate. Add markdown and line breaks to improve docs.rs formatting. ## `arithmeticHandler` Add markdown formatting to the `arithmeticHandler` to aid clarity. Also added an example, though I've set it to `ignore` as by default this will be run as part of `cargo test` and I didn't want to change anything but documentation.