[Docs] Align _cpython.mojo docs with the C API

[helehex]

Where is the problem?

https://github.com/modularml/mojo/blob/main/stdlib/src/python/_cpython.mojo

What can we do better?

The _cpython module provides bindings to CPython, so it should reflect and wrap it in a consistent way, with corresponding documentation. Currently some things are missing though.

This involves:

• Links to the official documentation, or docstrings which accurately describe the function
• Documenting cases where a function raises

Maybe this could fall under bindgen and be done automatically:
We could put the ref comments in and later pull in the documentation automatically and put it in as a docstring.
However, the documentation does not seem very gen-friendly.

Anything else?

This was talked about in the public stdlib discussion here)
@soraros started work on cleaning it up here

[soraros]

We should also use types (c_char etc.) from ffi in the get_function calls.