v5: document how to calculate filter_age_16384ths am: 851efcf150 am: 71096e0c8b

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

Change-Id: I81bebf0fb418a56a4debccda08e6b731bee7c808
Signed-off-by: Automerger Merge Worker <[email protected]>