Document the cursor pagination on ListDatasetElements

Both ListProcessElements and ListDatasetElements use a custom cursor-based pagination that includes an optional with_count boolean parameter, defaulting to False. In the API response, count is null unless with_count is set to True.

In ListDatasetElements, the cursor parameter is missing, and the default page parameter is here instead, so the endpoint probably cannot be paginated using the API client. with_count is also missing.

This is due to the way the paginator is built: since there is no pagination_class attribute at all, DRF deduces the default pagination class should be used for the API schema instead of the modified cursor pagination. This does not affect ListProcessElements since it uses pagination_class directly. We'll need to update some parts of CustomCursorPagination so that it does not take any arguments and instead uses the conventional method of customizing pagination through inheritance as DRF documents.

count should also be documented as nullable in both endpoints. Some code already exists to try to do that, but it does not work. We could try to use a discriminator in the API response to document that count is null or an integer depending on with_count, on top of a help text.