Math: adapt to CORRADE_NODISCARD changes.

Yeah, this is how it should have been since the beginning.

src/Magnum/Math/Swizzle.h

@@ -149,11 +149,11 @@ Use @ref scatterInto() to update the vector in-place.
     @ref Vector4::rgb(), @ref Vector4::xy(), @ref Vector3::xy()
 */
 #ifdef DOXYGEN_GENERATING_OUTPUT
-template<char ...components, class T> CORRADE_NODISCARD constexpr T scatter(const T& vector, const Vector<sizeof...(components), typename T::Type>& values)
+template<char ...components, class T> CORRADE_NODISCARD("use scatterInto() to scatter in-place") constexpr T scatter(const T& vector, const Vector<sizeof...(components), typename T::Type>& values)
 #else
 /* Using std::common_type otherwise GCC 4.8 fails to match the arguments
    in SwizzleTest::scatterOneComponent() */
-template<char ...components, class T> CORRADE_NODISCARD constexpr T scatter(const T& vector, const typename std::common_type<Vector<sizeof...(components), typename T::Type>>::type& values)
+template<char ...components, class T> CORRADE_NODISCARD("use scatterInto() to scatter in-place") constexpr T scatter(const T& vector, const typename std::common_type<Vector<sizeof...(components), typename T::Type>>::type& values)
 #endif
 {
     return Implementation::scatterRecursive<T, sizeof...(components), components...>(vector, values, 0);