[goocanvas] Document the need to set :has-tooltip on the Canvas

[Murray Cumming <murrayc src gnome org>]

commit 1a672ec7d5b53ddbd4b53b4d7a3f9e9fc2be5b2b
Author: Will Thompson <will thompson collabora co uk>
Date:   Thu Oct 7 15:51:26 2010 +0100

    Document the need to set :has-tooltip on the Canvas
    
    https://bugzilla.gnome.org/show_bug.cgi?id=631614

 src/goocanvasitem.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)
---
diff --git a/src/goocanvasitem.c b/src/goocanvasitem.c
index 57db899..205f7b4 100644
--- a/src/goocanvasitem.c
+++ b/src/goocanvasitem.c
@@ -516,6 +516,15 @@ goo_canvas_item_base_init (gpointer g_iface)
 								 FALSE,
 								 G_PARAM_READWRITE));
 
+      /**
+       * GooCanvasItem:tooltip:
+       *
+       * The tooltip to display for the item, or %NULL to display no tooltip.
+       *
+       * Note that this property has no effect unless the
+       * #GtkWidget:has-tooltip property is set to %TRUE on the #GooCanvas
+       * containing this item.
+       */
       g_object_interface_install_property (g_iface,
 					   g_param_spec_string ("tooltip",
 								_("Tooltip"),