UpdateEngineStable: document security difference am: 4aae5e1436

Original change: https://android-review.googlesource.com/c/platform/system/update_engine/+/2889550

Change-Id: I135da1733efd3c73dcddfa1db52321c036748358
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>