Should roxygen warn when S3 methods are not tagged with @export?

[lionel-]

It seems like it would help people with internal generics where it is not clear why you need to register. Cf r-lib/devtools#2293.

[hadley]

I think the biggest question is who's responsibility this is. Should it be roxygen2, devtools, or (maybe) usethis? I guess roxygen2 already has all the machinery to figure out if a function is a S3 method, so it probably makes the most sense here. It could be part of the namespace roclet that would just warn if it spotted unexported methods.