Updates guidance re. usage of ARIA tags for legends for radio and checkbox fieldsets.

[klepas]

Description

See the sister/brother PR in the UI-Kit: https://github.com/AusDTO/gov-au-ui-kit/pull/457

Definition of Done

• [x] Content/documentation reviewed by Jools or someone in the #content-design team
• [ ] Code reviewed by one of the core developers
• [x] Acceptance Testing
  • [x] HTML5 validation (CircleCI)
  • [x] Accessibility testing & WCAG2 compliance
• [ ] Stakeholder/PO review

[AndrewArch]

change suggested matches reported screen reader bug/issue and acknowledged solution