[docs] add note about inability to access file system at runtime

[dummdidumm]

Closes #8039
I'm not sure what else besides documentation we can do here - the runtime error is outside of our control, and simulating this during dev sounds super hard (there's also an open issue about that somewhere I think).

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@sveltejs/adapter-cloudflare-workers	Patch
@sveltejs/adapter-netlify	Patch
@sveltejs/adapter-vercel	Patch

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[Rich-Harris]

To be fair we could include files in lambda deployments on netlify and vercel if we wanted to, we would just to figure out a good way to make it happen (that didn't, for example, mean including the files in every lambda if using split: true). But until then this makes sense