Mention in documentation that it is available since 3.16
https://bugzilla.gnome.org/show_bug.cgi?id=767165
* @GDK_GRAB_NOT_VIEWABLE: the grab window or the @confine_to window are not
* viewable.
* @GDK_GRAB_FROZEN: the resource is frozen by an active grab of another client.
- * @GDK_GRAB_FAILED: the grab failed for some other reason.
+ * @GDK_GRAB_FAILED: the grab failed for some other reason. Since 3.16
*
* Returned by gdk_device_grab(), gdk_pointer_grab() and gdk_keyboard_grab() to
* indicate success or the reason for the failure of the grab attempt.