API Review: clearUserDataOnFailedRestore manifest flag

Document the bahavior chane for the old flag (clearUserData) starting
from API 29. The API level check is already in place: https://cs.corp.google.com/android/frameworks/base/services/backup/java/com/android/server/backup/UserBackupManagerService.java?q=UserBackupManagerService.java&sq=package:android&g=0&l=1521

Bug: 127465240
Test: N/A
Change-Id: Ic9c497857427a72e89e5e1a0937b3502d8623f21