Document in comments that wxAuiToolBarArt element sizes use DIPs

This is not as good as relying on the type system, but better than
nothing.

include/wx/aui/auibar.h

@@ -318,6 +318,8 @@ public:
                          wxWindow* wnd,
                          const wxAuiToolBarItem& item) = 0;
 
+    // Note that these functions work with the size in DIPs, not physical
+    // pixels.
     virtual int GetElementSize(int elementId) = 0;
     virtual void SetElementSize(int elementId, int size) = 0;
 
@@ -431,6 +433,7 @@ protected:
     wxPen m_gripperPen2;
     wxPen m_gripperPen3;
 
+    // These values are in DIPs and not physical pixels.
     int m_separatorSize;
     int m_gripperSize;
     int m_overflowSize;