Skip D401 for docstrings that start directly with a section.

[anntzer]

Skip D401 for docstrings that start directly with a section. Closes #463.

Please make sure to check for the following items:

• [X] Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior, you should probably add or change a test case file under tests/test_cases/ and add it to the list under tests/test_definitions.py.
  If you've added or changed a command line option, you should probably add or change a test in tests/test_integration.py.
• [X] Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

(afaict the test failure on py3.9 is spurious)

[anntzer]

Kindly bumping.