How is this different to normal `sleep`? Can the difference please be documented?

(Also, I believe a goal for `std::rt::io` is to be independent of the event loop that it is running on, so a function with a name like `sleep_uv` seems like it would be exposing too many implementation details of the `libuv` event loop. That said, I'm no expert on the RT, and I haven't looked that the implementations at all.)