Require private class attributes to be documented

They are already documented, so no point in failing on them now.

pyproject.toml

@@ -73,6 +73,7 @@ arg-type-hints-in-docstring = false
 baseline = ".config/pydoclint-baseline.txt"
 check-return-types = false
 exclude = '\.git|\.tox|build|out|venv'
+should-document-private-class-attributes = true
 show-filenames-in-every-violation-message = true
 skip-checking-short-docstrings = false
 style = "google"