AP_Math: Add comments to quaternion methods

libraries/AP_Math/quaternion.cpp

@@ -534,6 +534,8 @@ void QuaternionT<T>::from_axis_angle_fast(const Vector3<T> &axis, T theta)
     q4 = axis.z * st2;
 }
 
+// create a quaternion by integrating an angular velocity over some time_delta, which is 
+// assumed to be small
 template <typename T>
 void QuaternionT<T>::from_angular_velocity(const Vector3<T>& angular_velocity, float time_delta)
 {

libraries/AP_Math/quaternion.h

@@ -112,6 +112,8 @@ public:
     // the axis vector must be length 1, theta should less than 0.17 radians (i.e. 10 degrees)
     void        from_axis_angle_fast(const Vector3<T> &axis, T theta);
 
+    // create a quaternion by integrating an angular velocity over some time_delta, which is 
+    // assumed to be small
     void        from_angular_velocity(const Vector3<T>& angular_velocity, float time_delta);
 
     // rotate by the provided rotation vector