Improve JavaDoc for FirewallRule constants am: 011a7f51c1 am: 740b38e626 am: d8281e4e61

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

Change-Id: Ief6f5b51b68bf7624fb706868f8f769c4f517949
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>