minor tweaks in the doc

<div id="comment:0"></div>

little details, mainly cosmetics

Component: **documentation**

Author: **Frédéric Chapoton**

Branch/Commit: **[`b3ab217`](https://github.com/sagemath/sagetrac-mirror/commit/b3ab217f4f610729b64738d5cc42053a85270d56)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34289_

