Merge "Mention device sleep behaviour for System.nanoTime" am: 439de6824b am: 63eb9b000c

Original change: https://android-review.googlesource.com/c/platform/libcore/+/2068947

Change-Id: I80301c445ce41d8dbd0e2c343bbc4a70d03b6949
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>