tweak our .lgtm file, add documentation

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by not checking the `all_*.py` files



CC:  @mkoeppe

Component: **refactoring**

Author: **Frédéric Chapoton**

Branch/Commit: **[`2457c05`](https://github.com/sagemath/sagetrac-mirror/commit/2457c05381f3c09923b9d5262f15716421ba4edc)**

Reviewer: **Matthias Koeppe**

_Issue created by migration from https://trac.sagemath.org/ticket/33867_

