[glib] gsettings: clarify changed signal documentation

[Matthias Clasen <matthiasc src gnome org>]

commit 0e5e3d0d65613f095a5365c827fcf1a4986b7716
Author: Lars Uebernickel <lars uebernickel canonical com>
Date:   Tue Jun 2 07:10:31 2015 +0200

    gsettings: clarify changed signal documentation
    
    Make it clear that this signal is only guaranteed to be emitted when a
    key has been read before.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=750257

 gio/gsettings.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gio/gsettings.c b/gio/gsettings.c
index 3e2ae44..0a50295 100644
--- a/gio/gsettings.c
+++ b/gio/gsettings.c
@@ -730,6 +730,9 @@ g_settings_class_init (GSettingsClass *class)
    * This signal supports detailed connections.  You can connect to the
    * detailed signal "changed::x" in order to only receive callbacks
    * when key "x" changes.
+   *
+   * Note that @settings only emits this signal if you have read @key at
+   * least once while a signal handler was already connected for @key.
    */
   g_settings_signals[SIGNAL_CHANGED] =
     g_signal_new (I_("changed"), G_TYPE_SETTINGS,