fix some details in doc of charpoly_frobenius

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and also pep8 standard for the code in this file

Component: **algebraic geometry**

Author: **Frédéric Chapoton**

Branch/Commit: **[`0e88192`](https://github.com/sagemath/sagetrac-mirror/commit/0e88192afda9bfd7c551276380bc2dd76feadbb8)**

Reviewer: **David Coudert**

_Issue created by migration from https://trac.sagemath.org/ticket/34619_

