docs: add section on update propagation

[Rich-Harris]

alternative to #14931

closes #14772

[changeset-bot]

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Latest commit: cbf8d6b

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[github-actions]

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pnpm add https://pkg.pr.new/svelte@14938

[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14938-svelte.vercel.app/

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