Docs: Mention differences between tseslint.config's extends and ESLint core's

I have looked for existing open or closed documentation requests that match my proposal.
I have read the FAQ and my problem is not listed.

Splitting out of #10725 (comment): we refreshed our docs on tseslint.config but didn't mention that eslint/rfcs#126 was accepted and eslint/eslint#19116 is in Implementing status.

We should probably mention this in some way, but I'm not clear on what the right phrasing is. 🤔

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