[issue41430] Document C docstring behavior

Change by James Corbett : -- keywords: +patch pull_requests: +20816 stage: -> patch review pull_request: https://github.com/python/cpython/pull/21673 ___ Python tracker ___ __

[issue41430] Document C docstring behavior

New submission from James Corbett : As described in https://stackoverflow.com/questions/25847035/what-are-signature-and-text-signature-used-for-in-python-3-4, https://bugs.python.org/issue20586, and https://stackoverflow.com/questions/50537407/add-a-signature-with-annotations-to-extension-met