Properly document the relationships between update(), sync_before_render(), and render()

[johannesugb]

...most importantly, document that in some cases, render-specific updates shall be made in render() if they overwrite previous frames' data or so. This is probably not obvious at all with the current state of the documentation.

update() shall be used for CPU-side updates and maybe also GPU-updates which are completely independent of the resources used #concurrent_frames() frames previously. But it may not be used for updates which depend on the resources from #concurrent_frames() frames before because the Fence which waits for GPU work to complete is waited-on in window::sync_before_render().

Definition of done:

• This behavior is documented in gvk::invokee, or at least a pointer is provided to gvk::window
• This behavior is documented in gvk::window
• This behavior is documented somewhere else properly - at least in README.md