We have here some inconsitency between the namings start_string vs start_line

If you specify start_line in the pyproject.toml, it will not be recognized correctly.
We have here some inconsistency in the code base itself, & also in the documentation.

Should be easy to solve:

1. Decide for which option to go: start_string or start_line
2. Unify the codebase
3. Adapt the docs accordingly