Document a couple of new-in-U features. am: 9e2120bde3

Original change: https://googleplex-android-review.googlesource.com/c/platform/bionic/+/22819376

Change-Id: I523a88b5e95e5f61f5625f773ab3b23923ec7c26
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>