Update API docs for Framebuffer, FramebufferCube to sync with in-line docs in regl.d.ts.
Update API docs for Framebuffer, FramebufferCube to sync with in-line docs in regl.d.ts.

- Add detail on the several ways of defining dimensions (`shape`, `radius`, `width`, `height` properties).
- Make FramebufferCube options table more similar to that of Framebuffer.
- Revise some wording for clarity.
- Provide GLenum types for ‘float’, ‘half float’.

regl-project/regl,regl-project/regl,regl-project/regl