[gtkmm] Plug: Add documentation

[Murray Cumming <murrayc src gnome org>]

commit 7d31cf28e23e3b22f11a7915e31e039376055372
Author: Murray Cumming <murrayc murrayc com>
Date:   Wed Oct 9 11:06:23 2013 +0200

    Plug: Add documentation

 gtk/src/plug.hg |   23 ++++++++++++++++++++++-
 1 files changed, 22 insertions(+), 1 deletions(-)
---
diff --git a/gtk/src/plug.hg b/gtk/src/plug.hg
index f7f31cb..bbf93d3 100644
--- a/gtk/src/plug.hg
+++ b/gtk/src/plug.hg
@@ -31,7 +31,28 @@ namespace Gtk
 //gtkx.h (needed to get GtkPlug and GtkSocket) pulls this in,
 //assuming that you will be careful when doing so.
 
-//TODO: Documentation
+/** Toplevel for embedding into other processes.
+ *
+ * Together with Gtk::Socket, Gtk::Plug provides the ability to embed
+ * widgets from one process into another process in a fashion that is
+ * transparent to the user. One process creates a Gtk::Socket widget
+ * and passes the ID of that widget's window to the other process,
+ * which then creates a Gtk::Plug with that window ID. Any widgets
+ * contained in the Gtk::Plug then will appear inside the first
+ * application's window.
+ *
+ * The communication between a Gtk::Socket and a Gtk::Plug follows the
+ * XEmbed protocol. This protocol has also been implemented in other toolkits,
+ * e.g. Qt, allowing the same level of
+ * integration when embedding a Qt widget
+ * in GTK+ or vice versa.
+ *
+ * <note>
+ * The Gtk::Plug and Gtk::Socket widgets are only available when GTK+ and gtkmm
+ * are compiled for the X11 platform and %GDK_WINDOWING_X11 is defined.
+ * They can only be used on a GdkX11Display.
+ * </note>
+ */
 class Plug : public Window
 {
   _CLASS_GTKOBJECT(Plug, GtkPlug, GTK_PLUG, Gtk::Window, GtkWindow)