Math: mention planeEquation() in Frustum docs and vice versa.

src/Magnum/Math/Frustum.h

@@ -87,7 +87,13 @@ template<class T> class Frustum {
         /** @brief Construct a frustum without initializing the contents */
         explicit Frustum(NoInitT) noexcept: _data{Vector4<T>{NoInit}, Vector4<T>{NoInit}, Vector4<T>{NoInit}, Vector4<T>{NoInit}, Vector4<T>{NoInit}, Vector4<T>{NoInit}} {}
 
-        /** @brief Construct a frustum from plane equations */
+        /**
+         * @brief Construct a frustum from plane equations
+         *
+         * The equations are in a form @f$ ax + by + cz + d = 0 @f$. You can
+         * use @ref planeEquation() to calculate the coefficients from a normal
+         * and a point.
+         */
         constexpr /*implicit*/ Frustum(const Vector4<T>& left, const Vector4<T>& right, const Vector4<T>& bottom, const Vector4<T>& top, const Vector4<T>& near, const Vector4<T>& far) noexcept: _data{left, right, bottom, top, near, far} {}
 
         /**

src/Magnum/Math/Vector4.h

@@ -245,7 +245,7 @@ using a dot product with the normal @f$ \boldsymbol{n} @f$ using the point
     d = - \boldsymbol{n} \cdot \boldsymbol{p}
 @f]
 @see @ref planeEquation(const Vector3<T>&, const Vector3<T>&, const Vector3<T>&),
-    @ref dot()
+    @ref dot(), @ref Frustum
 */
 template<class T> Vector4<T> planeEquation(const Vector3<T>& normal, const Vector3<T>& point) {
     return {normal, -Math::dot(normal, point)};