[#42][Doc] adjust documentation, fix trackball camera radius issue

Before:
If more than one trackball camera controller is added to your application, changing the m_radius variable via scroll event would lead to adjusting the radius for all cameras attached to the trackball camera controller.

After:
Before updating the radius, the m_radius variable is changed in respect to the current distance from the camera to its center point. This enables handling all cameras with attached trackball camera controller separately.