doc: add a sub-paragraph for outlined invalid radio buttons

[MaxLardenois]

Note: Please transform - [ ] into - (NA) in the description when things are not applicable

Related issues

closes #2947

Description

Add a paragraph for outlined invalid Radio buttons

Types of change

• Bug fix (non-breaking which fixes an issue)

Live previews

• https://deploy-preview-2953--boosted.netlify.app/docs/0.3/forms/radio-button/#outlined-invalid

Checklist

Contribution

• I have read the contributing guidelines

Accessibility

• My change follows accessibility good practices; I have at least run axe

Design

• My change respects the design guidelines defined in Orange Design System
• My change is compatible with a responsive display

Development

• My change follows the developer guide
• I have added JavaScript unit tests to cover my changes
• I have added SCSS unit tests to cover my changes

Documentation

• My change introduces changes to the documentation and/or I have updated the documentation accordingly

Checklist (for Core Team only)

• My change introduces changes to the migration guide
• My new component is well displayed in Storybook
• My new component is compatible with RTL
• Manually run BrowserStack tests
• Manually test browser compatibility with BrowserStack (Chrome >= 60, Firefox >= 60 (+ ESR), Edge, Safari >= 12, iOS Safari, Chrome & Firefox on Android)
• Code review
• Design review
• A11y review

After the merge

• Manually launch Percy tests

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