Clarify meaning of pose in *ObjectKinematics messages

Closes AutowareAuto#1014 (closed).

The issue was that it was unknown what the "pose" means, especially since the Shape can fully define the object – e.g. is the pose supposed to be the frame in which the shape is defined, or does the pose designate a special point on the object? In both cases, what should the orientation be?

Now, a comment clarifies that the pose designates the CoG and the orientation availability is reported accurately.