http://docs.unity3d.com/Manual/Script-SelectableTransition.html
When you say “missing” is there anything pointing to it? Anyways, the correct URL is: Redirecting to latest version of com.unity.ugui
This page, and there may be other broken links there: Redirecting to latest version of com.unity.ugui
Thanks, Google also got me there.
We have fixed many broken links, and will publish the updated version soon. Thanks for the report!