Backported PyCapsule from 3.1, and converted most uses of
CObject to PyCapsule.
diff --git a/Doc/c-api/concrete.rst b/Doc/c-api/concrete.rst
index 0595788..5ee611b 100644
--- a/Doc/c-api/concrete.rst
+++ b/Doc/c-api/concrete.rst
@@ -100,6 +100,7 @@
    descriptor.rst
    slice.rst
    weakref.rst
+   capsule.rst
    cobject.rst
    cell.rst
    gen.rst