docs: updated based on labelling and field descriptions mixins

[nikkimk]

Description

Updates documentation and planning guidance to reflect the gen2 mixin architecture:

• LabellingMixin handles labels and accessible names.
• FieldDescriptionMixin handles help text, error messages, and accessible descriptions.
• Historical controller references remain only where they explain the architectural decision.
• These changes do not apply to 1st-gen implementations.

No executable code changes.

Motivation and context

Labelling and field descriptions use mixins rather than a controller. This update removes outdated guidance and clarifies each mixin's responsibilities.

Related issue(s)

• Fixes SWC-2637

Screenshots (if appropriate)

Not applicable; documentation-only changes.

---

Author's checklist

• I have read the CONTRIBUTING and PULL_REQUESTS documents.
• I have reviewed the applicable accessibility practices.
• I have added automated tests to cover my changes.
• I have included a changeset if my change needs to be published.
• I have included updated documentation.

No runtime tests or changeset are required for these documentation-only changes.

---

Reviewer's checklist

• Includes a Jira ticket number without a link.
• Updated wording accurately describes the gen2 mixin responsibilities.
• Remaining controller references are intentionally historical.
• Documentation clearly excludes 1st-gen implementations.

Manual review test cases

• Verify terminology and responsibilities.

  1. Search case-insensitively for labellingController.
  2. Confirm any remaining references describe historical architecture, not current guidance.
  3. Confirm labels and accessible names belong to LabellingMixin, while help text and error messages belong to FieldDescriptionMixin.
  4. Confirm the changes apply only to gen2.

• Verify documentation formatting.

  1. Review the changed documents.
  2. Run applicable formatting and documentation checks.
  3. Confirm the updated content renders correctly.

Device review

Not applicable; no UI or runtime changes.

Accessibility testing checklist

Keyboard and screen reader testing are not applicable because this PR changes documentation only.

[changeset-bot]

⚠️ No Changeset found

Latest commit: 80d9bde

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR

[github-actions]

📚 Branch Preview Links

• Storybook (Gen2, dev mode)
• Storybook (Gen2, prod mode)
• Documentation Site (Gen1)
• Storybook (Gen1)

🔍 Gen1 Visual Regression Test Results

When a visual regression test fails (or has previously failed while working on this branch), its results can be found in the following URLs:

• Spectrum | Light | Medium | LTR
• Spectrum | Dark | Large | RTL
• Express | Light | Medium | LTR
• Express | Dark | Large | RTL
• Spectrum-two | Light | Medium | LTR
• Spectrum-two | Dark | Large | RTL
• High Contrast Mode | Medium | LTR

Deployed to Azure Blob Storage: pr-6832

If the changes are expected, update the current_golden_images_cache hash in the circleci config to accept the new images. Instructions are included in that file.
If the changes are unexpected, you can investigate the cause of the differences and update the code accordingly.

[5t3ph]

Updates look good, thanks for doing this!