Clarify that only diffs are rendered to IO every frame

[kdheepak]

Currently, when people read "immediate mode rendering", there's a chance that people think ratatui draws every pixel to the terminal. However that is not case, only diffs are rendered to IO (e.g. stdout). Currently, I believe diff is only mentioned in under-the-hood section.

It should probably more prominently mentioned in the README, docs and website.

[joshka]

To some extent this is true - the rendering happens from widget to memory to the buffer, but that's not the expensive bit (the actually copying to screen is).

[kdheepak]

I clarified the issue to mean rendering to IO.