Document indices in obs/action_set

[AntoinePrv]

Pull request checklist

• [ ] I have opened an issue to discuss the proposed changes: Fix #XX.
• [ ] I have modified/added tests to cover the new changes/features.
• [x] I have modified/added the documentation to cover the new changes/features.
• [x] I have ran the tests, checks, and code formatters.

Proposed implementation

Documentation for variables indices usage in obseravtions / action_set. Only the doc-string were modified.

[gasse]

@AntoinePrv can you look at those fixes, then re-open and re-merge as needed ? Sorry the review came a bit late.

[AntoinePrv]

Thanks for the review! I've fixed it in latest.