docs: add docs on state_unsafe_mutation error

[dummdidumm]

closes #14752

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14932-svelte.vercel.app/

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pnpm add https://pkg.pr.new/svelte@14932

[Rich-Harris]

I think it would be helpful to use an example where the mutated state is independent of the derived — it's easy to look at the current example and think 'well of course you can't update the thing the result depends on', when actually it's nothing to do with the cyclicality.

I'd guess that most people are encountering a non-cyclical version of this problem (i.e. treating the derived as a convenient place to update some independent state). Ideally the example would reflect real cases — @MathiasWP do you recall what sort of things led to you encountering this error?

[MathiasWP]

I think it would be helpful to use an example where the mutated state is independent of the derived — it's easy to look at the current example and think 'well of course you can't update the thing the result depends on', when actually it's nothing to do with the cyclicality.

I'd guess that most people are encountering a non-cyclical version of this problem (i.e. treating the derived as a convenient place to update some independent state). Ideally the example would reflect real cases — @MathiasWP do you recall what sort of things led to you encountering this error?

I've gone on a scavenge hunt to try and find the code that caused this, but with no luck (it was quite difficult to find in the first place when it happened) :/