Fix some doctest warnings

[mkoeppe]

These are some of the last remaining doctest warnings as of 10.3.beta7, as seen e.g. in https://github.com/sagemath/sage/pull/37250/files "Unchanged files with check annotations".

The ones not fixed here will need a solution for:

• # 32-bit / # 64-bit tag breaks block-scoped doctest tags #36099

📝 Checklist

• The title is concise, informative, and self-explanatory.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation accordingly.

⌛ Dependencies

[dcoudert]

the CI indicates a doctest warning in src/sage/geometry/polyhedron/backend_field.py line 93 that could be fixed in this PR: Variable 'p' referenced here was set only in doctest marked '# needs sage.rings.number_field sage.symbolic'

[mkoeppe]

Indeed, thanks.

[github-actions]

Documentation preview for this PR (built with commit 9bfc29a; changes) is ready! 🎉

[dcoudert]

LGTM.

[mkoeppe]

Thank you!