Add comments to explain the difference between offload.o and offload@mainline.o. am: 7738877dbb am: 6c3b9f6f20

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3205170

Change-Id: I9ba0ca4685bd72f10a9a8fa805e89c6e935b7b01
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>