Clarify potential underflow behavior in apf_interpreter docs am: 0106938c57 am: e564bae55a

Original change: https://android-review.googlesource.com/c/platform/hardware/google/apf/+/3147896

Change-Id: I5a8bc3956479784af3064b6ab343796961ff7c3d
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>