docs: add clarification on static template expressions

[Ocean-OS]

As mentioned in #15289, #14854, #14803, #13525, #13054, #9860 #11012 and #10629, there is a bit of confusion on what expressions are reactive in your template. This adds some clarification to the docs.

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Preview: https://svelte-dev-git-preview-svelte-15292-svelte.vercel.app/

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Playground

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[Rich-Harris]

If it's coming up this frequently then I suspect we should just fix it, even if that means slightly less optimal code in a few cases. Ideally we'd still optimise cases where we know for certain that there's no reactivity (e.g. a primitive value that's never reassigned, or is only reassigned with another primitive value), though that would mean adding some more information to bindings (which is something I've been wanting to do for a while, as I suspect it would pay dividends elsewhere as well)