Use new R6 roxygen support
In https://github.com/stan-dev/cmdstanr/issues/78#issuecomment-553179118 @stevebronder linked to the announcement of support for R6 documentation in roxygen2. This is good news! We should update.
https://www.tidyverse.org/blog/2019/11/roxygen2-7-0-0/#r6-documentation
Unfortunately it doesn't yet support documentation for methods added via $set(). The new release of roxygen2 only supports inline doc for methods defined when the class is initially defined.
We could either change our code to define all methods initially with the class or wait for a roxygen2 that supports what we need (it's on their to-do list: https://github.com/r-lib/roxygen2/issues/931).
I don't think it's worth changing to this at this point. It's still not quite what we need, so would take a lot of work to fit the cmdstanr doc into this format.