bpo-37487: Fix PyList_GetItem index description. (GH-14623)

0 is a legal index.

Doc/c-api/list.rst

@@ -59,9 +59,9 @@ List Objects
 .. c:function:: PyObject* PyList_GetItem(PyObject *list, Py_ssize_t index)
 
    Return the object at position *index* in the list pointed to by *list*.  The
-   position must be positive, indexing from the end of the list is not
-   supported.  If *index* is out of bounds, return *NULL* and set an
-   :exc:`IndexError` exception.
+   position must be non-negative; indexing from the end of the list is not
+   supported.  If *index* is out of bounds (<0 or >=len(list)),
+   return *NULL* and set an :exc:`IndexError` exception.
 
 
 .. c:function:: PyObject* PyList_GET_ITEM(PyObject *list, Py_ssize_t i)

Misc/NEWS.d/next/Documentation/2019-07-06-17-19-26.bpo-37487.QagfZ5.rst

+Fix PyList_GetItem index description to include 0.