Remove `_PyObject_GetDictPtr` from capi document

[Locked-chess-official]

Documentation

This function starts with _ but exists in capi document. Maybe it is needed to change the name or remove this document.

[StanFromIreland]

It is publicly exposed in our headers, and the documentation was added later in 8d37c62. We shouldn't just remove it.

You'll have to open an issue in https://github.com/capi-workgroup/problems/issues to promote it to "fully" public (i.e., to drop the underscore).