Add some documentation to spline_mapping.h.

PiperOrigin-RevId: 601161487
Change-Id: I75101a0f25180939f098f70f60ca8cbca49913f6

mjpc/planners/gradient/spline_mapping.h

@@ -25,11 +25,27 @@ namespace mjpc {
 // ----- spline constants ----- //
 inline constexpr int kMinGradientSplinePoints = 1;
 inline constexpr int kMaxGradientSplinePoints = 25;
-inline constexpr int kMinGradientSplinePower = 1;
-inline constexpr int kMaxGradientSplinePower = 5;
 
 // matrix representation for mapping between spline points and interpolated time
-// series
+// series.
+// A spline is made of num_input points, and each has one associated time, and
+// `dim` associated parameters.
+// The time series has num_output entries, each associated with a time and with
+// dim associated values.
+//
+// For sampling policies, we have
+// dim = model->nu
+// num_input = num_spline_points
+// num_output = trajectory_length
+//
+// The mapping is a matrix, A, of shape (dim*num_output) x (dim*num_input),
+// which can be used to go from spline parameters to the values of the sampled
+// time series, assuming a fixed set of times.
+//
+// Given a vector of containing spline parameters, v (length=dim*num_input),
+// flattened so that the parameters for each spline point are next to each
+// other, A*v gives the corresponding interpolated values, sampled at
+// output_times.
 class SplineMapping {
  public:
   // constructor