autodoc: extract function signature from docstring
When autodoc is used for documenting C extensions, it does not include the function signature (presumably because the Python signature cannot be extracted from a .so module).
Tools for generating C extensions (like Sphinx or Cython) do know the Python signature and should be able to make this information available to sphinx as well.
Cython already suppports embedding the signature in the first line of the docstring. It would be great if autodoc could try to read the function signature from there if it cannot be determined from the function itself.