Math: reference intersection functions from Frustum docs.

src/Magnum/Math/Frustum.h

@@ -48,7 +48,9 @@ namespace Implementation {
 
 Stores camera frustum planes in order left (index `0`), right (index `1`),
 bottom (index `2`), top (index `3`), near (index `4`) and far (index `5`).
-@see @ref Magnum::Frustum, @ref Magnum::Frustumd
+@see @ref Magnum::Frustum, @ref Magnum::Frustumd,
+    @ref Geometry::Intersection::pointFrustum(),
+    @ref Geometry::Intersection::boxFrustum()
 */
 template<class T> class Frustum {
     public: