gh-38513: various details in arithgroup_generic
various minor details in the modified file
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
URL: https://github.com/sagemath/sage/pull/38513
Reported by: Frédéric Chapoton
Reviewer(s): Matthias Köppe
