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

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



Change-Id: I69bbe95babd2d2fd9c3b29e9bb0552183e4cf316
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>