Document that wxMSW wxSpinButton supports inversed range.

This was surprising to discover and seems useful to know in the documentation,
if only to say that this is not supported under the other platforms.

interface/wx/spinbutt.h

@@ -168,6 +168,12 @@ public:
     /**
         Sets the range of the spin button.
 
+        In portable code, @a min should be less than or equal to @a max. In
+        wxMSW it is possible to specify minimum greater than maximum and the
+        native control supports the same range as if they were reversed, but
+        swaps the meaning of up and down arrows, however this dubious feature
+        is not supported on other platforms.
+
         @param min
             The minimum value for the spin button.
         @param max