Doxygen description edits for Joint, including new gifs

The usual distinction between a Gimbal joint and a Ball joint would be that gimbals use Euler angles and thus have a singularity, while balls use quaternions and are singularity-free. This description (which I think is correct) says that OpenSim ball joints also use Euler angles. If that is right, then the same singularity comment that appears for gimbal ought to be repeated here. (Better would be to switch to a singularity-free Ball, but that's a different issue.)