Clutter 0.8 and 1.0 are currently slotted. But they both try to install their gtk-doc documentation in the same locations /usr/share/gtk-doc/{clutter,cogl}/ So either the slotting should be removed (which would be sad), or they should be fixed to install the doc in {clutter,cogl}-{0.8,1.0}
clutter-0.8 is not in-tree; you must be using some overlay which gives that to you.