docs: React Aria separator docs

[yihuiliao]

Closes

✅ 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:

[rspbot]

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[dannify]

Is this meant to be at the bottom?

[yihuiliao]

@dannify Yup, there's no render props so the last thing that should appear is the default classname. Other doc pages are like this too

[rspbot]

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• View the storybook
• View the S2 storybook
• View the v3 docs
• View the S2 docs
• View the React Aria docs

[rspbot]

Build successful! 🎉

• View the storybook
• View the S2 storybook
• View the v3 docs
• View the S2 docs
• View the React Aria docs

[snowystinger]

@dannify i agree, it's a little hard to find the default class name, especially in other pages where it's sandwiched between two prop tables, it's so small. We should make it a little more prominent across the board I think.