Add explanation of metalava usages am: 9aa18f2b8b am: af40e049e8 am: c791237da8

Original change: https://android-review.googlesource.com/c/platform/tools/metalava/+/2046148

Change-Id: I3fe3c9ec142cf7aba2e2f9e4643b3adcaee0f823
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>