[wpimath] Document ChassisSpeeds::Discretize() math (NFC) (#6509)

wpimath/src/main/native/include/frc/kinematics/ChassisSpeeds.h

@@ -61,8 +61,15 @@ struct WPILIB_DLLEXPORT ChassisSpeeds {
                                   units::meters_per_second_t vy,
                                   units::radians_per_second_t omega,
                                   units::second_t dt) {
+    // Construct the desired pose after a timestep, relative to the current
+    // pose. The desired pose has decoupled translation and rotation.
     Pose2d desiredDeltaPose{vx * dt, vy * dt, omega * dt};
+
+    // Find the chassis translation/rotation deltas in the robot frame that move
+    // the robot from its current pose to the desired pose
     auto twist = Pose2d{}.Log(desiredDeltaPose);
+
+    // Turn the chassis translation/rotation deltas into average velocities
     return {twist.dx / dt, twist.dy / dt, twist.dtheta / dt};
   }