fix: digest D212 and D213 docstring

[12rambau]

Fix #60

Parse the docstring and split the first line from the others. This ensures that whatever convention is chosen (D212 or D213) the rest of the docstring keeps the right indentation.

Commit checklist:

• add tests that fail without the patch
• ensure all tests pass with pytest
• add documentation to the relevant docstrings or pages
• add versionadded or versionchanged directives to relevant docstrings
• add a changelog entry if this patch changes code