sagemathgh-38513: various details in arithgroup_generic

    
various minor details in the modified file


### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: sagemath#38513
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe