[3.6] bpo-30059: Include Py_Ellipsis in C API documentation (GH-1018) (GH-1149)

diff --git a/Doc/c-api/slice.rst b/Doc/c-api/slice.rst
index a825164..8b695e0 100644
--- a/Doc/c-api/slice.rst
+++ b/Doc/c-api/slice.rst
@@ -56,3 +56,14 @@
    .. versionchanged:: 3.2
       The parameter type for the *slice* parameter was ``PySliceObject*``
       before.
+
+
+Ellipsis Object
+---------------
+
+
+.. c:var:: PyObject *Py_Ellipsis
+
+   The Python ``Ellipsis`` object.  This object has no methods.  It needs to be
+   treated just like any other object with respect to reference counts.  Like
+   :c:data:`Py_None` it is a singleton object.