Document what the remaining <utility> includes are for.

Need to get rid of them in Math.

src/Magnum/Audio/Buffer.h

@@ -31,7 +31,7 @@
  */
 
 #include <climits>
-#include <utility>
+#include <utility> /* std::pair */
 
 #include <Corrade/Containers/ArrayView.h>
 

src/Magnum/Math/Algorithms/KahanSum.h

@@ -29,7 +29,7 @@
  * @brief Function @ref Magnum::Math::Algorithms::kahanSum()
  */
 
-#include <utility>
+#include <utility> /* std::declval :( */
 
 namespace Magnum { namespace Math { namespace Algorithms {
 

src/Magnum/Math/Functions.h

@@ -34,7 +34,7 @@
 
 #include <cstdlib> /* std::div() */
 #include <type_traits>
-#include <utility>
+#include <utility> /* std::pair */
 #include <Corrade/Utility/Move.h>
 #include <Corrade/Utility/StlMath.h>
 

src/Magnum/Math/Vector.h

@@ -29,7 +29,7 @@
  * @brief Class @ref Magnum::Math::Vector, function @ref Magnum::Math::dot(), @ref Magnum::Math::angle()
  */
 
-#include <utility>
+#include <utility> /* std::pair */
 #ifndef CORRADE_NO_DEBUG
 #include <Corrade/Utility/Debug.h>
 #endif

src/Magnum/PixelStorage.h

@@ -30,7 +30,7 @@
  */
 
 #include <cstddef>
-#include <utility>
+#include <utility> /* std::pair */
 
 #include "Magnum/Magnum.h"
 #include "Magnum/Math/Vector3.h"