AV1 RT: describe AV1E_SET_MV_COST_UPD_FREQ Off mode

Change-Id: Icae3fb60d8962b54184526bb794852c8f2888d3c

aom/aomcx.h

diff --git a/aom/aomcx.h b/aom/aomcx.h
index 362115c..6a9d628 100644
--- a/aom/aomcx.h
+++ b/aom/aomcx.h
@@ -1137,6 +1137,7 @@
    * 0: Update at SB level (default)
    * 1: Update at SB row level in tile
    * 2: Update at tile level
+   * 3: Off
    */
   AV1E_SET_MV_COST_UPD_FREQ,