Switch to using roxygen2-4.0-generated documentation

Roxygen2 4.0 is currently available as a pre-release from [klutometis/roxygen](https://github.com/klutometis/roxygen) and will soon be available on CRAN. While the upgrade is pretty painless, there are two changes to note:

* ``@S3method`` is now deprecated and so has been switched for ``@export``
* Rd files are no longer wrapped (this can be changed by setting ``Roxygen: list(wrap = FALSE)`` in the DESCRIPTION file