docs: document sign convention difference between MR and Featherstone in ABA

Author: ferrolho

src/ModernRoboticsBook.jl

@@ -1936,6 +1936,9 @@ function forward_dynamics_aba(
         ci[i] = ad(Vi[i+1]) * Ai[i] * joint_velocities[i]
         Gi = SMatrix{6,6}(spatial_inertias[i])
         IA[i] = Gi
+        # Featherstone's velocity-product force is p_A = v ×* (I v), where ×* is the
+        # spatial force cross product. In the Modern Robotics convention, ad(V)' gives
+        # the *negative* of the force cross product, so we negate: pA = -ad(V)' * G * V.
         pA[i] = -ad(Vi[i+1])' * Gi * Vi[i+1]
     end