Doc api

[martinkilbinger]

Summary

Developers should provide a summary of the proposed changes here and include "closes #" if this addresses an open issue.

Reviewer Checklist

Reviewers should tick the following boxes before approving and merging the PR.

• The PR targets the develop branch
• The PR is assigned to the developer
• The PR has appropriate labels
• The PR is included in appropriate projects and/or milestones
• The PR includes a clear description of the proposed changes
• If the PR addresses an open issue the description includes "closes #"
• The code and documentation style match the current standards
• Documentation has been added/updated consistently with the code
• All CI tests are passing
• API docs have been built and checked at least once (if relevant)
• All changed files have been checked and comments provided to the developer
• All of the reviewer's comments have been satisfactorily addressed by the developer