Review BDV related docstrings

[ehrenfeu]

Since we're now having automatic ruff linting enabled through a GitHub Action, it's much easier to follow the conventions.

Particularly the recent changes in the bdv module and the related tests are triggering complaints.

Docstrings should be added or fixed, making sure that:

• the summary line is concise and allows to understand the idea
• more details are given in the followup section, if necessary
• especially for tests-docstrings it is important to explain the test-flow (i.e. the idea of what the test does) properly, as this is sometimes extremely difficult to guess and understand from the test code itself

Current complaints can be seen in the details of the lint workflow, for example from run 13853569840.