There are several parts of the docs where it is only written "function" as a type itself. But it not enough precise. Each "function" type should be replaced with a typed function, including the arguments and their types (and the expected value).

For instance, on the page numbering setting, it is only written that it can accept a "function". I think that it should also add the following indications : (current_page: integer, total_pages: integer) -> text or something similar. It would avoid to have to manually guess what the arguments are, and I think that the purpose of the documentation is to make the objects understandable easily and clearly.