Documentation 
Section: GtkMessageDialog (C gtk+) / gtk.MessageDialog (pygtk)
It's simply a convenience widget; you could construct the equivalent of gtk.MessageDialog from gtk.Dialog without too much effort, but gtk.MessageDialog saves time.

Correct version:
It's simply a convenience widget; you could construct the equivalent of gtk.MessageDialog from gtk.Dialog without too much effort, but gtk.MessageDialog saves time.

One difference between generic Dialog and MessageDialog is that the latter by default does not appear in the taskbar. To override this behaviour, use gtk.gdk.Window.set_skip_taskbar_hint().

Other information:
I think it is good to add because:
1) one would otherwise expect consistency between a Dialog and MessageDialog, a subclass of Dialog
2) obtaining this consistency is not totally trivial (it involves a gdk method)

I reported the text and proposed fix for the python bindings, but I checked it also applies  to the C documentation.