More documentation about TETHERING_VIRTUAL am: 50e486668c am: 54ec4adcc9

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

Change-Id: I241f4453bb29a76cd19d05ce3b0f71a589a3c724
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>