Add documentation about ART Service. am: bb7c41c476 am: 1d21821b1f

Original change: https://android-review.googlesource.com/c/platform/art/+/2612930

Change-Id: I3fd202c34f99ea1c35d7813abb6277ff991711ba
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>