Document that Refresh() always erases background in non-MSW ports

The distinction between redrawing the background and the rest of the
window only exists in MSW and we can't refresh the window without
refreshing its background in the other ports, but we can at least
document this.

Closes #19234.

interface/wx/window.h

@@ -2202,7 +2202,8 @@ public:
         should use Update() instead.
 
         @param eraseBackground
-            If @true, the background will be erased.
+            If @true, the background will be erased too. Note that in non-MSW
+            ports background is always erased.
         @param rect
             If non-@NULL, only the given rectangle will be treated as damaged.