Expand renderIntoDocument migration guidance

[rinadavidyuk826-sudo]

Closes #28

Summary

• explain why renderIntoDocument was awkward to clean up in tests
• point readers to render, unmount, and cleanup in React Testing Library
• keep the migration guidance scoped to the existing warning page

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[github-actions]

Size changes

Details

📦 Next.js Bundle Analysis for react-dev

This analysis was generated by the Next.js Bundle Analysis action. 🤖

This PR introduced no changes to the JavaScript bundle! 🙌

[rinadavidyuk826-sudo]

I completed the CLA signing flow and got the confirmation that future pull requests from this GitHub account should now be accepted.

Could you please re-check the CLA status for this PR? Thanks.

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Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Meta Open Source project. Thanks!