gbytes: Clarify nullability for g_bytes_unref() in docs
gbytes: Clarify nullability for g_bytes_unref() in docs

https://bugzilla.gnome.org/show_bug.cgi?id=791318

johne53/MB3Glib,johne53/MB3Glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,endlessm/glib,johne53/MB3Glib,endlessm/glib,johne53/MB3Glib