More documentation about TETHERING_VIRTUAL am: 50e486668c

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3175321

Change-Id: I9e88a1e86bb8067376f320561520ff5feda69dee
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>