gh-132246: Add special buffer methods to C API Type Object docs (gh-132247)

Two special methods, __buffer__ and __release_buffer__ were added to
Python 3.12 by PEP 688. The C API Type Object documentation for slots
includes `tp_as_buffer`, and sub-slots `bf_getbuffer`, `bf_releasebuffer`
but does not refer to the Python Data Model version of those. Add the
missing references.