You don't dock to the combat frame. You dock to the general frame. They introduced a new Dock frame system which from my testing seems a bit incomplete but works great for the general frame.
If you hover the chat frames near the tab bar area of the chat frame area containing the general tab you should start seeing a small whitish line. This is the new DockManager widget. Now if you move the chat frame near any other chat frame not currently docked to the general frame and you won't see that widget appear.
Now, they have put in place a set of functions to create a new DockManager widget but at this time I haven't seen how you then get it to allow chat frames to dock with it. Most of the existing code hard codes to the GENERAL_CHAT_DOCK rather than use a passed variable to signify the DockManager frame to use.