Improve ZneOptions extrapolator documentation and add "fallback"

[ihincks]

What is the expected feature or enhancement?

At the moment the API docs for this class are a bit sparse on the details. This issue is to 1) add text that describes the various extrapolator methods and how their order matters and 2) add the new "fallback" method that will soon be supported.

cc @chriseclectic

Acceptance criteria