Getting some scaffold docs in place and making some naming adjustments
to improve consistency with Rust APIs and clarity.

- Render::Element -> Render::Output, and Render::Output is now only
required to be IntoElement instead of Element so we can return
components.
- RenderOnce::Element -> RenderOnce::Output
- Element::layout -> Render::request_layout - Clarify that this doesn't
actually *perform* layout, but simply requests

Release Notes:

- N/A