docs: clarify "obsolete" in runes blog post

[benmccann]

People continue to be confused by this section

We've had an FAQ about this forever, but not everyone who reads the blog post will see the FAQ
https://svelte-5-preview.vercel.app/docs/faq#breaking-changes-and-migration-which-things-will-be-deprecated-in-svelte-5

[changeset-bot]

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