Math: clearly document what the TypeForSize helper is for.

I was initially not sure if it has some other use than the swizzle APIs.
It doesn't, so mention that clearly.

src/Magnum/Math/Color.h

@@ -1942,6 +1942,7 @@ MAGNUM_EXPORT Debug& operator<<(Debug& debug, const Color4<UnsignedByte>& value)
 #endif
 
 namespace Implementation {
+    /* Used by Swizzle.h */
     template<class T> struct TypeForSize<3, Color3<T>> { typedef Color3<T> Type; };
     template<class T> struct TypeForSize<3, Color4<T>> { typedef Color3<T> Type; };
     template<class T> struct TypeForSize<4, Color3<T>> { typedef Color4<T> Type; };

src/Magnum/Math/Swizzle.h

@@ -59,6 +59,9 @@ namespace Implementation {
         template<class T> constexpr static T value(const Math::Vector<size, T>&) { return T(1); }
     };
 
+    /* Additional specializations in Vector2.h, Vector3.h, Vector4.h and
+       Color.h. That way it's possible to return a concrete Vector subtype
+       without having to include it directly from here. */
     template<std::size_t size, class T> struct TypeForSize {
         typedef Math::Vector<size, typename T::Type> Type;
     };

src/Magnum/Math/Vector2.h

@@ -250,6 +250,7 @@ _MAGNUM_VECTORn_OPERATOR_IMPLEMENTATION(2, Vector2)
 #endif
 
 namespace Implementation {
+    /* Used by Swizzle.h */
     template<std::size_t, class> struct TypeForSize;
     template<class T> struct TypeForSize<2, T> { typedef Math::Vector2<typename T::Type> Type; };
     #ifndef MAGNUM_NO_MATH_STRICT_WEAK_ORDERING

src/Magnum/Math/Vector3.h

@@ -288,6 +288,7 @@ _MAGNUM_VECTORn_OPERATOR_IMPLEMENTATION(3, Vector3)
 #endif
 
 namespace Implementation {
+    /* Used by Swizzle.h */
     template<class T> struct TypeForSize<3, T> { typedef Math::Vector3<typename T::Type> Type; };
     #ifndef MAGNUM_NO_MATH_STRICT_WEAK_ORDERING
     template<class T> struct StrictWeakOrdering<Vector3<T>>: StrictWeakOrdering<Vector<3, T>> {};

src/Magnum/Math/Vector4.h

@@ -315,6 +315,7 @@ template<class T> Vector4<T> planeEquation(const Vector3<T>& normal, const Vecto
 }
 
 namespace Implementation {
+    /* Used by Swizzle.h */
     template<class T> struct TypeForSize<4, T> { typedef Math::Vector4<typename T::Type> Type; };
     #ifndef MAGNUM_NO_MATH_STRICT_WEAK_ORDERING
     template<class T> struct StrictWeakOrdering<Vector4<T>>: StrictWeakOrdering<Vector<4, T>> {};