Fix types in buffer/memoryview docs (GH-98118)

The definition of obj in the `Py_buffer` struct is as a PyObject*
https://github.com/python/cpython/blob/ec091bd47e2f968b0d1631b9a8104283a7beeb1b/Include/pybuffer.hGH-L22

PyMemoryView_GET_BASE returns `.obj` - thus its return type
should be a PyObject* (or at least a void*). It definitely
doesn't return `Py_buffer`
(cherry picked from commit c459fedf7cfd5dadf72e088d789c7375b3a6e093)

Co-authored-by: da-woods <dw-git@d-woods.co.uk>