Update AUTOMATIC_EMERGENCY_BRAKING_ENABLED documentation in HAL

An implementation of AUTOMATIC_EMERGENCY_BRAKING_ENABLED should return
false when there's an issue with the ADAS system and it cannot retrieve
the property. It should not return NOT_AVAILABLE.

Bug: 264336118
Test: atest VtsHalAutomotiveVehicle_TargetTest
Change-Id: I2dab0c48865ed1ccc40433b9fbe5ea341a2488f3
diff --git a/automotive/vehicle/aidl_property/android/hardware/automotive/vehicle/VehicleProperty.aidl b/automotive/vehicle/aidl_property/android/hardware/automotive/vehicle/VehicleProperty.aidl
index 4a9799e..28deaf6 100644
--- a/automotive/vehicle/aidl_property/android/hardware/automotive/vehicle/VehicleProperty.aidl
+++ b/automotive/vehicle/aidl_property/android/hardware/automotive/vehicle/VehicleProperty.aidl
@@ -3429,6 +3429,10 @@
      * Set true to enable AEB and false to disable AEB. When AEB is enabled, the ADAS system in the
      * vehicle should be turned on and monitoring to avoid potential collisions.
      *
+     * IVehicle#get must not return any NOT_AVAILABLE value in StatusCode. Other StatusCode values
+     * like TRY_AGAIN may still be used as needed. For example, if AEB is not available because the
+     * vehicle speed is too low, IVehicle#get must return false.
+     *
      * This property is defined as read_write, but OEMs have the option to implement it as read
      * only.
      *