Document wxBitmapBundle::GetBitmap(wxDefaultSize) behaviour

The size parameter is explicitly allowed to be wxDefaultSize here,
meaning to return the bitmap in the default bundle size.

interface/wx/bmpbndl.h

@@ -315,6 +315,9 @@ public:
         cached, avoid calling it for many different sizes if you do use it, as
         this will create many bitmaps that will never be deleted and will
         consume resources until the application termination.
+
+        @param size The size of the bitmap to return, in physical pixels. If
+            this parameter is wxDefaultSize, default bundle size is used.
      */
     wxBitmap GetBitmap(const wxSize& size) const;