Update Py_HashPointer() doc

Author: vstinner

docs/api.rst

@@ -525,6 +525,8 @@ Python 3.13
 
    See `Py_HashPointer() documentation <https://docs.python.org/dev/c-api/hash.html#c.Py_HashPointer>`__.
 
+   Not available on the limited C API.
+
 .. c:function:: Py_hash_t PyObject_GenericHash(PyObject *obj)
 
    See `PyObject_GenericHash() documentation <https://docs.python.org/dev/c-api/hash.html#c.PyObject_GenericHash>`__.