Clarify potential underflow behavior in apf_interpreter docs am: 0106938c57

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

Change-Id: Icc62187da33b865a375d6d6e5bbbf248c55278db
Signed-off-by: Automerger Merge Worker <[email protected]>