Add JSDoc comments for error overlay to match other experimental flags

[delucis]

Changes

Add configuration docs for the experimental.errorOverlay flag to match those for prerender and contentCollections.

Currently in docs we render out those other flags but not the errorOverlay flag: https://docs.astro.build/en/reference/configuration-reference/#experimental-flags

Testing

n/a docs change only

Docs

Yes. Is there a reason we wouldn’t want to document this like we do other experimental features?

[changeset-bot]

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Latest commit: 4d8a942

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[sarah11918]

Approving the text on behalf of Docs Maintainers!