Document wxGridTable::GetValueAsCustom() return value constraint

The returned pointer must be allocated on the heap as
wxGridCellDateTimeRenderer::GetString() currently deletes it.

This seems like a rather poor idea, as any internally stored values must
be cloned on each call, but it seems too dangerous to change this as it
could result in massive memory leaks in the existing code.

interface/wx/grid.h

@@ -1633,6 +1633,11 @@ public:
 
         This should only be called if CanGetValueAs() returns @true when called
         with @a typeName. Default implementation always return @NULL.
+
+        Note that if the pointer is not null, it will be deleted by the caller,
+        so it must be allocated on the heap by any class overriding this
+        method. In practice, it means that the value stored internally must be
+        cloned on every call.
      */
     virtual void *GetValueAsCustom(int row, int col, const wxString& typeName);