Merge "Improve the comment for atomic_flag." into main am: c897cd7a6d

Original change: https://android-review.googlesource.com/c/platform/bionic/+/3533161

Change-Id: I7265a300696a8047b028ca414f837b1c8f5d0013
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>