Update comments for VehicleApPowerStateShutdownParam

Comments for _IMMEDIATELY parameters stated that shutdown cannot be postponed.
However it is possible that CPMS will send SHUTDOWN_POSTPONE if SHUTDOWN_PREPARE takes longer. Updated comments to remove that statement.

Bug: 319670846
Change-Id: I0d172ba5d24b86c7f2403acae05961eca72b0c2d
Test: manual build