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

@@ -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,