Added Robin Dunn's explanation on the relationship of wxBitmap and wxImage into interface doxygen section

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68196 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

interface/wx/image.h

@@ -314,6 +314,17 @@ const unsigned char wxIMAGE_ALPHA_OPAQUE = 0xff;
     the wxBitmap::wxBitmap(wxImage,int depth) constructor.
     This bitmap can then be drawn in a device context, using wxDC::DrawBitmap.
 
+    More on the difference between wxImage and wxBitmap: wxImage is just a
+    buffer of RGB bytes with an optional buffer for the alpha bytes. It is all
+    generic, platform independent and image file format independent code. It
+    includes generic code for scaling, resizing, clipping, and other manipulations
+    of the image data. OTOH, wxBitmap is intended to be a wrapper of whatever is
+    the native image format that is quickest/easiest to draw to a DC or to be the
+    target of the drawing operations performed on a wxMemoryDC. By splitting the
+    responsibilities between wxImage/wxBitmap like this then it's easier to use
+    generic code shared by all platforms and image types for generic operations and
+    platform specific code where performance or compatibility is needed.
+
     One colour value of the image may be used as a mask colour which will lead to
     the automatic creation of a wxMask object associated to the bitmap object.