Update BLIND_SPOT_WARNING_ENABLED documentation in HAL

An implementation of BLIND_SPOT_WARNING_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: 264946039
Test: atest VtsHalAutomotiveVehicle_TargetTest
Change-Id: I3f8765b9bbaa96931dd2ae0bab1c8fbc913b261c