Revert "Math: remove transitive include for Functions.h."

Nope, that was a very stupid change. The *vector* min() / max() is used
in intersect(), and this change broke the bindings build (likely along
with random other code).

This reverts commit 8c228c69c9. Partially,
keeping the extra added includes and also documenting why the
Functions.h are included.

src/Magnum/Math/Range.h

@@ -35,8 +35,7 @@
    all have it directly in <type_traits> because it just makes sense */
 #include <type_traits>
 
-/* Taking also scalar min()/max() from here, which is otherwise advertised in
-   Functions.h */
+#include "Magnum/Math/Functions.h" /* Vector min() / max() */
 #include "Magnum/Math/Vector3.h"
 
 #ifdef MAGNUM_BUILD_DEPRECATED