Add more sane python doc

Maik Nijhuisrequested to merge
add-more-sane-python-doc into main

Also fix an issue with pybind11_mkdoc:

It generates non-ISO C++ compliant macro code. With -Wpedantic, g++ gives this error: error: ISO C++11 requires at least one argument for the "..." in a variadic macro [-Werror]

Adding an extra comma in the generated code solves this issue.

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