The only place where "nullable" appears in the doc tree is... clinic.rst (in reference to places where a C pointer can be NULL, which *is* a reasonable use of the term).

There's another problem in the patch: if you define "nullable_int_t" and "nullable_Py_ssize_t" in a public header (longobject.h), then you probably need to prefix them with "_Py" (so: "_Py_nullable_int_t" and "_Py_nullable_ssize_t", for example). Same for the #define's.