Document dv_cost_upd_freq default in realtime only

Document that the default of dv_cost_upd_freq is COST_UPD_OFF in
realtime only builds.

Change-Id: I221ad3e08efe7c954ba598a0ca81b4e533288377

av1/av1_cx_iface.c

diff --git a/av1/av1_cx_iface.c b/av1/av1_cx_iface.c
index 756f5bc..4a31981 100644
--- a/av1/av1_cx_iface.c
+++ b/av1/av1_cx_iface.c
@@ -204,6 +204,7 @@
 // coeff_cost_upd_freq: COST_UPD_OFF
 // mode_cost_upd_freq: COST_UPD_OFF
 // mv_cost_upd_freq: COST_UPD_OFF
+// dv_cost_upd_freq: COST_UPD_OFF
 static const struct av1_extracfg default_extra_cfg = {
   7,  // cpu_used
   1,  // enable_auto_alt_ref