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

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

Change-Id: I76a0d14864625273038441821f347face47e4c04
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>