GObject: Elaborate notify emission timing, options

We only said receiving ::notify doesn’t always guarantee that anything
actually changed, but we didn’t explain how we can get that guarantee.

https://bugzilla.gnome.org/show_bug.cgi?id=795096

gobject/gobject.c

@@ -477,11 +477,17 @@ g_object_do_class_init (GObjectClass *class)
    * @gobject: the object which received the signal.
    * @pspec: the #GParamSpec of the property which changed.
    *
-   * The notify signal is emitted on an object when one of its
-   * properties has been changed. Note that getting this signal
-   * doesn't guarantee that the value of the property has actually
-   * changed, it may also be emitted when the setter for the property
-   * is called to reinstate the previous value.
+   * The notify signal is emitted on an object when one of its properties has
+   * its value set through g_object_set_property(), g_object_set(), et al.
+   *
+   * Note that getting this signal doesn’t itself guarantee that the value of
+   * the property has actually changed. When it is emitted is determined by the
+   * derived GObject class. If the implementor did not create the property with
+   * %G_PARAM_EXPLICIT_NOTIFY, then any call to g_object_set_property() results
+   * in ::notify being emitted, even if the new value is the same as the old.
+   * If they did pass %G_PARAM_EXPLICIT_NOTIFY, then this signal is emitted only
+   * when they explicitly call g_object_notify() or g_object_notify_by_pspec(),
+   * and common practice is to avoid doing so if the value remained unchanged.
    *
    * This signal is typically used to obtain change notification for a
    * single property, by specifying the property name as a detail in the