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

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



Change-Id: Ide9a8b81ff2b0689d7c488bb7df67cba5324aec2
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>