DOC: Docstring script to not require ending period for bullet points
- [X] closes #25461
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR tries to solve the issue #25461 .

I thought that the validator should check that the last line of the parameter description is a part of a bullet point before raising the error PR09.

In order to do that, I had to add an extra method in the Docstring class that return a parameter description line by line (in a list). This is because Docstring.parameter_desc() returns the description as 1 string.

Please let me know how I can improve this or if I did not consider something.