Updated vehicle documentation

Based on question at jrouwe/JoltPhysics.js#88

Jolt/Physics/Vehicle/VehicleEngine.h

@@ -34,7 +34,7 @@ public:
 	float					mMaxTorque = 500.0f;						///< Max amount of torque (Nm) that the engine can deliver
 	float					mMinRPM = 1000.0f;							///< Min amount of revolutions per minute (rpm) the engine can produce without stalling
 	float					mMaxRPM = 6000.0f;							///< Max amount of revolutions per minute (rpm) the engine can generate
-	LinearCurve				mNormalizedTorque;							///< Curve that describes a ratio of the max torque the engine can produce vs the fraction of the max RPM of the engine
+	LinearCurve				mNormalizedTorque;							///< Y-axis: Curve that describes a ratio of the max torque the engine can produce (0 = 0, 1 = mMaxTorque). X-axis: the fraction of the RPM of the engine (0 = mMinRPM, 1 = mMaxRPM)
 	float					mInertia = 0.5f;							///< Moment of inertia (kg m^2) of the engine
 	float					mAngularDamping = 0.2f;						///< Angular damping factor of the wheel: dw/dt = -c * w
 };

Jolt/Physics/Vehicle/WheeledVehicleController.h

@@ -31,8 +31,8 @@ public:
 	float						mInertia = 0.9f;							///< Moment of inertia (kg m^2), for a cylinder this would be 0.5 * M * R^2 which is 0.9 for a wheel with a mass of 20 kg and radius 0.3 m
 	float						mAngularDamping = 0.2f;						///< Angular damping factor of the wheel: dw/dt = -c * w
 	float						mMaxSteerAngle = DegreesToRadians(70.0f);	///< How much this wheel can steer (radians)
-	LinearCurve					mLongitudinalFriction;						///< Friction in forward direction of tire as a function of the slip ratio (fraction): (omega_wheel * r_wheel - v_longitudinal) / |v_longitudinal|. You can see slip ratio as the amount the wheel is spinning relative to the floor. 0 means the wheel has full traction and is rolling perfectly in sync with the ground, 1 is for example when the wheel is locked and sliding over the ground.
-	LinearCurve					mLateralFriction;							///< Friction in sideway direction of tire as a function of the slip angle (degrees): angle between relative contact velocity and vehicle direction
+	LinearCurve					mLongitudinalFriction;						///< On the Y-axis: friction in the forward direction of the tire. Friction is normally between 0 (no friction) and 1 (full friction) although friction can be a little bit higher than 1 because of the profile of a tire. On the X-axis: the slip ratio (fraction) defined as (omega_wheel * r_wheel - v_longitudinal) / |v_longitudinal|. You can see slip ratio as the amount the wheel is spinning relative to the floor: 0 means the wheel has full traction and is rolling perfectly in sync with the ground, 1 is for example when the wheel is locked and sliding over the ground.
+	LinearCurve					mLateralFriction;							///< On the Y-axis: friction in the sideways direction of the tire. Friction is normally between 0 (no friction) and 1 (full friction) although friction can be a little bit higher than 1 because of the profile of a tire. On the X-axis: the slip angle (degrees) defined as angle between relative contact velocity and tire direction.
 	float						mMaxBrakeTorque = 1500.0f;					///< How much torque (Nm) the brakes can apply to this wheel
 	float						mMaxHandBrakeTorque = 4000.0f;				///< How much torque (Nm) the hand brake can apply to this wheel (usually only applied to the rear wheels)
 };