docs: clear up relative position confusion

[snowystinger]

Closes #5094
followup to #8091 so that people hopefully don't forget the position relative in the future

We can't handle this in our RAC components and we can't handle it if someone uses VisuallyHidden on its own. So it makes sense that it should be documented

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

🧢 Your Project:

[snowystinger]

better section title? use inline alert?

[reidbarber]

Caveats?

[rspbot]

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[filipw01]

@snowystinger I think this gotcha has a bit different root cause than what you described.

I'm almost certain #5094 and #8081 are both caused by the issue I described here https://filip-wachowiak.vercel.app/blog/default-absolute-positioning-in-a-scroll-container

TLDR is that interactive elements inside a visually hidden class work weird when focused in a scroll container in chrome

[snowystinger]

I didn't go into details, but I'm happy to adjust it if you think there is a clearer message here.
One thing I didn't include is that the position relative element should be as close to the visual representation so that the VO and other assistive technologies virtual cursors appear near the visual representation as well.