Document better frame diffing

[Valentin271]

When building a frame, there is a diff algorithm that compares it to the previous one and only renders the difference.

I've answered this a few times on discord and probably on github. I think this is important (or at least interesting) and needs to be better documented.

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A few places that could be documented are

• https://docs.rs/ratatui/latest/ratatui/#drawing-the-ui (maybe just states it here, not explain it)
• https://docs.rs/ratatui/latest/ratatui/struct.Terminal.html (last paragraph)
• https://docs.rs/ratatui/latest/ratatui/struct.Terminal.html#method.draw
• https://docs.rs/ratatui/latest/ratatui/struct.Frame.html (maybe also render_widget/render_stateful_widget as this doc will be seen in-editor)

The only place I've seen this stated is here https://ratatui.rs/concepts/rendering/under-the-hood/#flush