Add docs for HelpText to CheckboxGroup and RadioGroup

[reidbarber]

Closes #3439

✅ 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:

RSP

[adobe-bot]

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[adobe-bot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[adobe-bot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[adobe-bot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation

[adobe-bot]

Build successful! 🎉

• View the storybook
• View the storybook-17
• View the storybook-16
• View the documentation