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

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

Change-Id: I43776feab2ae758859410bbb048ef28efc259939
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>