Merge "Doc updates for #apex and #systemapi" am: 357ebe5e am: 61734dfb am: b0c62e6c

Original change: https://android-review.googlesource.com/c/platform/build/soong/+/2407675



Change-Id: I34167bccdcf66642e620142d29fcb22d7d3c3470
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>