The iana-tunnel-type module has no errors or nits of any kind.
The ietf-extension-example module has no errors of any kind, and 1 nit.
The document appears to follow all YANG usage guidelines (but 1) correctly.

I have a minor comment about the example module. I guess it has to use <CODE BEGINS>.
(The <EXAMPLE BEGINS> discussion did not go anywhere).

This module looks a little too real to average users, especially after tools extract it
from the RFC and put it in the YangModels repo under the standard/ietf/RFC directory.

You should rename the ietf-extension-example module to example-iftunnel-extension
(or anything starting with "example-"). The module should not begin with "ietf-" unless it is a real module.