Add remark on getFrameClassicalAcceleration

include/pinocchio/algorithm/frames.hpp

@@ -125,6 +125,11 @@ namespace pinocchio
    *             This is different from the "spatial" acceleration in that centrifugal effects are accounted for.
    *             You must first call pinocchio::forwardKinematics to update placement, velocity and acceleration values in data structure.
    *
+   * @remark     In the context of a frame placement constraint \f$J(q) a + \dot{J}(q, v) v = 0\f$,
+   *             the classical acceleration of the frame corresponds to the \f$\dot{J}(q, v) v\f$ term.
+   *             It is significantly more efficient to call this function than,
+   *             say, to apply the matrix \f$\dot{J}(q, v)\f$ (from getFrameJacobianTimeVariation) to the velocity \f$v\f$.
+   *
    * @param[in]  model       The kinematic model
    * @param[in]  data        Data associated to model
    * @param[in]  frame_id    Id of the operational Frame