Uniformize headline: functions

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Component: **documentation**

Author: **Kwankyu Lee**

Branch/Commit: **[`8184039`](https://github.com/sagemath/sagetrac-mirror/commit/818403963bc01bec9ee5a9f38383009c8fcf4942)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/34574_

