Added doxygen for RobotState::attachBody() warning of danger.

This comment change highlights the danger of using the same AttachedBody instance with multiple RobotState instances.  It seems like a convenient thing to do, but it will give bad results, I'm almost certain.

The fact that I recommend that the user make a copy of the AttachedBody object, but there is no copy constructor for the AttachedBody class probably says we should change the API at least a bit.  Adding a copy constructor would be one way.  There are probably other ways.

In the meantime, documenting the existing behavior seems worthwhile.