C API docs: Py_IncRef and Py_DecRef are not documented as functions

[encukou]

Documentation

Py_IncRef and Py_DecRef are mentioned in prose at the end of Reference Counting docs. They should be documented as C functions so they show up in the index and can be linked.

[erlend-aasland]

Fixed by:

• gh-91755: Document Py_IncRef and Py_DecRef as C function. #91805
• [3.11] gh-91755: Document Py_IncRef and Py_DecRef as C functions (GH-91805) #92916
• [3.10] gh-91755: Document Py_IncRef and Py_DecRef as C functions (GH-91805) #92917