Doc that the value cannot be NULL.

Author: markshannon

Doc/c-api/tuple.rst

@@ -87,7 +87,7 @@ Tuple Objects
 .. c:function:: void PyTuple_SET_ITEM(PyObject *p, Py_ssize_t pos, PyObject *o)
 
    Like :c:func:`PyTuple_SetItem`, but does no error checking, and should *only* be
-   used to fill in brand new tuples.
+   used to fill in brand new tuples. Both ``p`` and ``o`` must be non-``NULL``.
 
    Bounds checking is performed as an assertion if Python is built in
    :ref:`debug mode <debug-build>` or :option:`with assertions <--with-assertions>`.