Improve JavaDoc for FirewallRule constants am: 011a7f51c1

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/2046447

Change-Id: I318f2a08beacd7ec96ff74b2a8ddc493fee86a65
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>