This is just the order of their declaration in the VCL manual.

As a side effect it works around a bug where the sess.xid syntax
requirements would prevent sess.timeout_idle to be used in VCL 4.0,
which is less intrusive than a proper fix.

The bug was fixed in trunk without being noticed in the first place
after many heavy changes to libvcc. For a stable branch this is less
risky than a back-port since there are only two sess.* symbols.

Fixes #3564