Update add-diagnostic.rst

[Sylviabohnenstengel]

Description

Fixes #

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• [x] Documentation has been updated to reflect change.
• [ ] New code has tests, and affected old tests have been updated.
• [x] All tests and CI checks pass.
• [x] Added an entry to the top of docs/source/changelog.rst
• [ ] Conda lock files have been updated if dependencies changed.
• [x] Marked the PR as ready to review.

[github-actions[bot]]