Improve documentation for MotionType to serve scripting users

[tkuchida]

The MotionType documentation lists enums but does not provide the mapping to unsigned integer that scripting users need. The first entry in the table is 0 by default but it would be clearer if the numbers were listed (compare to the analogous table in the Coord documentation).

• Here's an example Coord definition:

  opensim-core/OpenSim/Simulation/SimbodyEngine/BallJoint.h

  Line 79 in a01cf54

  enum class Coord: unsigned {

• Here's the MotionType definition:

  opensim-core/OpenSim/Simulation/SimbodyEngine/Coordinate.h

  Line 110 in ab5bccb

  enum MotionType

[tkuchida]

Fixed by #3365 🦎