stop saying don't move root joint

docs/animation/skeleton_kinematics.md

@@ -92,7 +92,7 @@ which is a simple extension actually. Since each term is independent and added t
 
 You should implement multi-target IK, which will take a `vector` of `IK_Handle*`s called `active_handles` which stores the information a target point for a joint. See `scene/skeleton.h` for the definition of `IK_Handle` structure.
 
-In order to implement this, you should update `Joint::compute_gradient` and `Skeleton::step_ik`. `Joint::compute_gradient` should calculate the gradient of <img src="task2_media/0079.png" style="height:18px"> in the x,y, and z directions, and add them to `Joint::angle_gradient` for all relevant joints. `Skeleton::step_ik` should actually do the gradient descent calculations and update the `pose` of each joint. In this function, you should probably use a very small timestep, but do several iterations (say, 10s to 100s) of gradient descent in order to speed things up. For even faster and better results, you can also implement a variable timestep instead of just using a fixed one. Note also that the root joint should never be updated.
+In order to implement this, you should update `Joint::compute_gradient` and `Skeleton::step_ik`. `Joint::compute_gradient` should calculate the gradient of <img src="task2_media/0079.png" style="height:18px"> in the x,y, and z directions, and add them to `Joint::angle_gradient` for all relevant joints. `Skeleton::step_ik` should actually do the gradient descent calculations and update the `pose` of each joint. In this function, you should use a small timestep, but do several iterations (say, 10s to 100s) of gradient descent in order to speed things up. For even faster and better results, you can also implement a variable timestep instead of a fixed one. Finally, also note that the gradient descent should never effect `Skeleton::base_pos`.
 
 A key thing for this part is to _remember what coordinate frame you're in_, because if you calculate the gradients in the wrong coordinate frame or use the axis of rotation in the wrong coordinate frame your answers will come out very wrong!