gh-37551: minor details + non-recursive algo in integer_valued_polynomials
This adds a better algo (non-recursive) for the shift of variables in
integer-valued polynomial rings.
Also some little tweaks to the doc in this file.
### :memo: Checklist
- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [x] I have created tests covering the changes.
- [x] I have updated the documentation accordingly.
URL: https://github.com/sagemath/sage/pull/37551
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Martin Rubey
