add clarification for focus response

- Focus response should be always sent when there is a
  focus request.
- Otherwise, focus request will time-out and will
  leads into focus reset.

bug: 27874358
Change-Id: I9e55d0183e9b0623871103107f2e7a475d4808c3
(cherry picked from commit 972dbb9e4ff75380bac35a8f24d1a013f6290184)
diff --git a/include/hardware/vehicle.h b/include/hardware/vehicle.h
index b7deb89..67d8155 100644
--- a/include/hardware/vehicle.h
+++ b/include/hardware/vehicle.h
@@ -465,6 +465,11 @@
  *                   LOSS_TRANSIENT: always should be VEHICLE_AUDIO_EXT_FOCUS_CAR_TRANSIENT
  *   int32_array[3]: should be zero.
  *
+ * A focus response should be sent per each focus request even if there is no change in
+ * focus state. This can happen in case like focus request only involving context change
+ * where android side still needs matching focus response to confirm that audio module
+ * has made necessary changes.
+ *
  * If car does not support VEHICLE_PROPERTY_AUDIO_FOCUS, focus is assumed to be granted always.
  *
  * @value_type VEHICLE_VALUE_TYPE_INT32_VEC4