Since we launched the new Ruby API docs last year we told people to submit issues in the GitHub repo that contains the generated docs.
But we are currently experimenting with accepting pull requests to the stubs repo; https://github.com/SketchUp/ruby-api-stubs
(More info: https://github.com/SketchUp/ruby-api-docs/issues/32)
If that seems to work, then we might make that repo the main one to report documentation issues. Right now things are in some flux as we experiment with various tools and solutions.