documentation/python: implement name mapping for _all__.

documentation/python: implement name mapping for _all__.

Basically, if there's this in the code:

    from ._native import Foo as PublicName
    from ._native import sub as submodule

    __all__ = ['PublicName', 'submodule']

then it's clear that the docs should refere library.PublicName and
library.submodule instead of library._native.Foo / library._native.sub.
This mapping is done for both pybind11 signatures and pure Python code.