Math: document why certain Range constructors are impossible to add.

I think I tried this at least once in the past already, and failed the
same way. So just document that it's missing to prevent me from wasting
time on this again in the future.

src/Magnum/Math/Range.h

@@ -477,6 +477,9 @@ template<class T> class Range2D: public Range<2, T> {
          */
         constexpr /*implicit*/ Range2D(const Containers::Pair<Vector<2, T>, Vector<2, T>>& minmax) noexcept: Range<2, T>{minmax.first(), minmax.second()} {}
 
+        /* Cannot have a constructor from X and Y range as
+           Range2D{{0, 1}, {2, 3}} would then be ambiguous :/ */
+
         /** @copydoc Range(const Range<dimensions, U>&) */
         template<class U> constexpr explicit Range2D(const Range2D<U>& other) noexcept: Range<2, T>(other) {}
 
@@ -609,6 +612,13 @@ template<class T> class Range3D: public Range<3, T> {
         /** @copydoc Range(const VectorType&, const VectorType&) */
         constexpr /*implicit*/ Range3D(const Vector3<T>& min, const Vector3<T>& max) noexcept: Range<3, T>(min, max) {}
 
+        /* A Range2D  constructor from X and Y range would cause
+           Range2D{{0, 1}, {2, 3}} to be ambiguous, thus for consistency
+           there's no Range3D constructor from a X, Y and Z range even though
+           it wouldn't be ambiguous. Similarly there is no constructor from a
+           XY and Z range because then Range3D{{}, {1, 2}} would be ambiguous
+           as well :/ */
+
         /**
          * @brief Construct a range from a pair of minimal and maximal coordinates
          * @m_since{2020,06}