docs: note that $derived memoizes output

[dummdidumm]

closes #14772

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14931-svelte.vercel.app/

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[Rich-Harris]

I think we should put this information in a separate appendix-y section, alongside 'Understanding dependencies' — it feels more like an interesting aside than a necessary piece of information. The only time it should affect outcomes is when dealing with mutated stores, per #14772. I feel like that's enough of an edge case that it probably doesn't need a call-out here, but if we decided that it did then we definitely don't want to clutter up the top section with that information. Opened #14938 as an alternative

[Rich-Harris]

merged #14938, so closing