Improve JavaDoc for FirewallRule constants am: 011a7f51c1 am: 04b2f3c7df

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

Change-Id: I1e2690d4c3782c0e916c3374134b4e1872e8a08b
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>