Merge "<sys/_system_properties.h>: improve documentation." into main am: 5b22f93ab9

Original change: https://android-review.googlesource.com/c/platform/bionic/+/3194644

Change-Id: I0b70924eb25fbbfc13477ed70c97fb544a64fc24
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>