[Typing] Add code docs also to type stubs

[jpfeuffer]

Currently, we write it to the pxd only. But since pyi typestubs overwrite signatures and docs for most IDEs, we should additionally put the docs to the typestubs. Maybe we can even remove them from the pxds but this can be optional for now (I did not think it through).

[jpfeuffer]

Unfortunately, documentation tools (e.g. Sphinx) do not support type stubs well, so we should keep docstrings in pxd for now.