v5: document how to calculate filter_age_16384ths am: 851efcf150

Original change: https://android-review.googlesource.com/c/platform/hardware/google/apf/+/2986090

Change-Id: Iea6bd68c02524d8a79c4c37d1c11f66946d0b4ff
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>