Document that wxDC::Clear() uses white background by default

This officially documents the historical behaviour of this method in
wxMSW and wxGTK2.

interface/wx/dc.h

@@ -261,6 +261,9 @@ public:
         Note that SetBackground() method must be used to set the brush used by
         Clear(), the brush used for filling the shapes set by SetBrush() is
         ignored by it.
+
+        If no background brush was set, solid white brush is used to clear the
+        device context.
     */
     void Clear();