Improve JavaDoc for FirewallRule constants am: 011a7f51c1

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

Change-Id: Ic6717db3b44abf9e7c3c4cb4720a44b7a7e259ac
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>