docs: Clarify use of graphene_ray_t
The ray type is useful for raycasting in order to do hit testing in 3D
space.
* #graphene_ray_t is a structure representing a ray emitted by an origin,
* identified by a point in 3D space, in a given direction, identified
* by a vector with 3 components.
+ *
+ * A common use of #graphene_ray_t is ray-casting to find which objects in
+ * a 3D scene are under the coordinates of the pointer.
*/
#include "graphene-private.h"
