Signal bone_setup_changed remains undocumented. I took a quick look at
the cpp code, but its purpose remained unclear to me. If anyone can
steer me in the right direction, I'm happy to flesh this out.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
